Báo cáo khoa học: "hybridization within the genus Quercus"

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Review article hybridization within the genus Quercus L Natural BS Rushton Department of Biological and Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland, BT52 1SA, UK Summary — Hybridization within the genus Quercus L appears to be extensive and reports vary sightings of individual hybrid trees to small numbers of individual hybrid trees within populations from to populations with characteristics of small-scale (eg Q robur and Q petraea in Hurepoix, France) and large-scale introgression (eg Q robur and Q petraea in Scotland) and, in some cases, the occur- rence of hybrid swarms (eg Q douglasii and Q turbinelia subsp californica in California). This has persuaded some authorities to question the current formal species concept in the genus and to sug- gest alternatives. The evidence supporting these cases of hybridization is examined in detail. The majority of the re- ports of hybrids between species of Quercus are based on an analysis of morphological data alone using a variety of univariate, bivariate and, more effectively, multivariate statistics, while other forms of evidence, such as estimates of fertility in the putative hybrids, resynthesis of hybrids, habitat char- acteristics of the putative hybrids and F segregation of parental types, have only been used occa- 2 sionally. Data from chemotaxonomic investigations of suspected Quercus hybrids (mainly isozymes and phenolic components) in some instances support the morphological evidence but in other in- stances are contradictory; chemical data are also shown to be variable and possibly related to envi- ronmental variation which will limit their usefulness. It is concluded that, before any radical revision of the genus is attempted in which the specific limits are redefined, a wider application of the possible techniques for the study of hybrids be applied in or- der to clarify the true extent of gene flow between Quercus species. hybridization / introgression / chemotaxonomy / morphology / Quercus L natural Résumé — Hybridation à l’intérieur du genre Quercus L. L’hybridation à l’intérieur du genre Quercus L est très largement répandue. Les descriptions d’hybrides concernent soit des arbres iso- lés, soit un nombre limité d’arbres situés en peuplement (Q robur et Q petraea à Hurepoix, France), soit des zones d’introgression (Q robur et Q petraea en Écosse), soit de larges populations gré- gaires d’hybrides (Q douglasii et Q turbinella subsp californica en Californie). La notion même d’es- pèce à l’intérieur du genre a été mise en doute par les spécialistes, qui ont suggéré d’autres interpré- tations. Les différents cas d’hybridation sont examinés en détail dans cette contribution. La majorité d’entre eux se réfère à des données morphologiques interprétées sous forme univariée, bivariée ou multivariée. Par contre d’autres méthodes de mise en évidence telles que les estimations de fertilité des hybrides, les hybridations contrôlées, les ségrégations des types parentaux en F et la descrip- , 2 tion de l’habitat des hybrides putatifs, ont été plus rarement utilisées. Les données chimiotaxonomi- ques relatives aux hybrides suspectés (essentiellement isozymes et composés phénoliques) corro- borent les observations morphologiques dans certains cas, mais les infirment dans d’autres cas. Les
caractères biochimiques manifestent également des variations liées au milieu, qui limitent leur utilisa- tion. En conclusion, il est recommandé d’utiliser l’ensemble des techniques disponibles pour l’étude de l’hybridation et des flux géniques avant de remettre en cause de manière radicale le genre Quercus. hybridation naturelle / introgression / chimiotaxonomie / morphologie / Quercus L THE DETECTION OF HYBRIDIZATION INTRODUCTION Hybridization manifests itself in a number It is estimated that Quercus L, of the one of ways, but the initial recognition of hy- largest genera of flowering plants, includes brids is by morphological intermediacy, the about 450 species (Jones, 1974), although putative hybrids showing evidence of inter- the literature contains considerably more mediate character states or a combination names and descriptions than this and vari- of suspected parental character states able estimates for the total number of spe- (Phipps, 1984). Indeed, as Gottlieb (1972) cies. Recorded hybrids between these points out, in the absence of morphological would appear to be both common and intermediacy, hybridity would not be sus- widespread. The earliest record of a hybrid pected. When the parental species are suf- oak in America was the description of x ficiently distinct, morphology alone may be Q hispanica by Michaux in 1812 (Palmer, sufficient to establish a case for hybridity, 1948). In Europe, there are many similar but where, as is often the case in Quercus, early records (see Gardiner, 1974). The the parental species show a wide range of apparent abundance of hybrids in certain natural variation and/or possesses few di- areas has caused taxonomic confusion agnostic characters, other criteria have to (and "complete frustration"; Tucker, 1961) be used. These include (Gottlieb, 1972): 1) in the past and, in certain floras, has un- an additive biochemical profile for charac- doubtedly led to misidentification. ters, such as flavonoids or proteins, which Population studies have indicated that are present in one or other parent but not the pattern of hybridization may follow 2 in both; 2) unusual amounts of interpopula- distinct paths: 1) the population shows evi- tional morphological variation (resulting dence of hybrid swarm formation, where from segregation of parental differences); the majority of the population appears 3) the occurrence of the putative hybrid in completely intermediate between the 2 intermediate habitats and evidence that suspected parental species; or 2) the pop- the putative hybrid has intermediacy for ulation shows evidence of introgression physiological characters; 4) the occurrence (Anderson, 1953), where the population of the putative hybrid in areas where the 2 consists of one species and a series of F 1 suspected parents are sympatric; 5) the and backcrossed hybrids. Wigston (1974) occurrence of the putative hybrid in geo- has reviewed the essential characteristics logical strata more recent than either of the of introgression and how they apply to 2 suspected parents; 6) the existence of at Quercus. least partial fertility in F hybrids between 1 This paper reviews the evidence which the parents to permit the possible produc- has been utilized in the detection of hy- tion of segregant genotypes; and 7) experi- brids and provides an evaluation, so far as mental production of individuals that re- current knowledge allows, of the different semble the putative hybrid in segregants of types of evidence. hybrids between the parents.
be more easily discriminated species could These criteria are broadly the same as in one season than in another; the general- those proposed by Stace (1980) and Craw- ity of this result needs to be confirmed (see ford (1985) and build on those already es- also Blue and Jensen, 1988). tablished in the earlier part of this century (see Stace, 1975). To this list may be add- In the early population studies, the stan- ed the possibility of reduced fertility shown dard approach was to construct hybrid indi- by some hybrids and DNA polymorphism. ces based on a limited range of morpho- logical characters and display these data Within Quercus, few examples exist in in the form of bivariate scatter diagrams in which a thorough investigation using all the which the 2 axes of variation represented above criteria has been completed. quantitative characters and each point on the scatter was usually a tree (Cousens, 1963, 1965). The points were annotated to PATTERNS OF MORPHOLOGICAL VARIATION show the variation in characters expressed in hybrid-index form to produce, for each point, a metroglyph which encapsulated Morphological intermediacy is the major, the variation pattern (eg Brophy and Par- and often only, criterion used in assessing nell, 1974). While this approach has much the status of putative oak hybrids. Charac- to commend it, since the full pattern of the ters are usually restricted to leaf and fruit- variation is expressed together, the inter- ing structures, though others (eg, buds: pretation may be problematic because of Jensen, 1988; bark: Dupouey, 1983) have the difficulties in choosing appropriate been utilized. The comparative uniformity quantitative characters for the axes (Rush- of floral structures within the genus (and ton, 1978). possibly their ephemeral nature) has limit- Subsequently, with the advent of numer- ed their use in population studies. Restric- ical taxonomic methods, multivariate meth- tion of samples to only fruiting specimens odologies were utilized and a wide range inevitably underestimates levels of hybridi- of these have now found application in ty. In addition, differences in fruit produc- analysis of morphological data from oak tion from year to year similarly bias sam- populations, including principal compo- pling, if samples are restricted to only nents analysis (Rushton, 1978, 1983; Du- fruiting individuals. pouey and Le Bouler, 1989; Jensen, 1989, has been the most im- Leaf morphology etc), discriminant function analysis (Ledig discriminator for oak taxa, both at portant et al, 1969; Rushton, 1974; Wigston, 1975; the level of the subgenus and the species Jensen et al, 1984) and cluster analysis (Muller, 1942), but clearly leaf morphology (Rushton, 1978; Jensen, 1988). These is subject to environmental modification. In ob- much methods have enabled more a the field, standardized collecting points in jective approach pattern-seeking to mor- (Cousens, 1963) have been used to over- phological data and sophisticated shape- come these effects. However, in a study of describing methods are now being evaluat- the influence of crown position on leaf ed (Jensen, 1990; Jensen et al, 1991) as a characters of Q palustris and Q velutina, means of collecting objective morphologi- Ludlam and Jensen (1989) concluded that cal data from oak leaves. "leaves should be collected from several One major disadvantage of these ap- positions on each tree and these collec- proaches (and earlier methods) is that of tions pooled for evaluating among-tree fixing known reference points to aid in in- variation". One further result was that the 2
about whether morphological terpretation but this has been overcome by generalize data overestimate or underestimate levels of reference populations .(com- the use posed of natural populations showing no of hybridity. signs of hybridity or artificial populations of herbarium specimens) which are used in all analyses (see fig 1; and Rushton, POLLEN VIABILITY 1978). In some oak taxa, different groups of re- Stace (1975) provides cautionary advice searchers have come to substantially dif- concerning the use of fertility of putative ferent conclusions regarding the levels of hybrids as an indicator of hybrid status, hybridity using morphological data. This is since it has been shown that hybrids may particularly true of the 2 wide-ranging, be completely sterile, or show no signifi- common European species, Q robur and cant reduction in fertility compared with the Q petraea (see below) and prompted Gar- be intermediate. However, parents, or diner (1970) to describe the discrepancies have been shown to pos- hybrids many "hybrid controversy". However, rarely as a sess reduced pollen viability and correla- the data sets directly comparable with are tion between morphological characteristics variation in sample sizes, numbers and and pollen viability is supportive evidence types of characters, methods of scoring for hybridity, eg Cercidium and Parkinsonia and analysis, use of reference material, (Carter, 1974; Carter and Rem, 1974). De- etc. It must also be borne in mind that spite the extensive investigations of mor- many species within the genus are ex- phological variation in Quercus spp, de- tremely variable in morphological charac- tailed studies of pollen viability are scant teristics and are also likely to show varia- and restricted to a very narrow range of tion in ability to cross, thus leading to species. However, in those studies in differential hybridization levels in different which extensive estimates have been areas. made, the general conclusion is that re- Consideration of the use of morphologi- duced pollen viability can frequently be ob- cal data to detect oak hybrids would indi- served in putative Quercus hybrids (see cate: 1) that considerably more attention also the discussion in Tucker, 1963; be paid to within-tree variation and possi- p 706-707). Of course, if substantive pol- bly between-season variation: and 2) that len sterility is a feature of Quercus hybrids, attempts should be made to standardize then this may limit gene flow between spe- methods of scoring and data analysis. Un- cies and promote the maintenance of spe- doubtedly, replicate samples from the cies identity. same trees, combined with population Surveys of Q robur and Q petraea in samples and analyzed using multivariate England and Wales (see fig 2; and Rush- methodologies would enable levels of phe- ton, 1978) and in Northern Ireland (Rush- notypic plasticity to be assessed alongside ton, 1988) have shown that morphological population variation, though the number of intermediacy is accompanied by a tenden- instances in which such intensive sam- cy for reduced pollen viability and Olsson pling has been coupled with extensive (1975a) has provided similar results for the sampling is very small. Where morphologi- same species. However, close examina- cal data have been collected alongside tion of assumed F hybrids indicated that 1 other data (see below), the correspon- they had an "unexpectedly high percent- dence between the different types of evi- age of pollen stainability" (Olsson, 1975a), dence may be poor, and it is difficult to
similar to that recorded in some intermedi- also variation between individual trees, though detailed data on the extent of this ate trees observed by Rushton (1978) and were not provided. Guttman and Weigt Minihan and Rushton (1984), and this was (1989) examined leaf material from 10 spe- tentatively ascribed to cryptic structural hy- cies of subgenus Erythrobalanus and 8 bridity. species of subgenus Quercus and were able to resolve 18 loci which represented 12 different enzyme systems. Subgeneric CHEMOTAXONOMIC STUDIES differences were re-enforced by the allo- zymes and species relationships com- pared well with other data, though there Electrophoretic evidence were differences. For example, Q nigra and Q laurifolia were found to be closely Crawford (1985) has pointed out that allo- related using morphological data analyzed zymes offer several advantages over other by cladistic and phenetic means (Jensen, types of evidence when assessing hybrid 1983), but isozymic results (Guttman and origin, since it is possible to examine the Weigt, 1989) suggested they were more products of a number of genes without the distantly related and in different groups problem of character intermediacy or addi- within the subgenus. One interesting con- tivity which is sometimes the case with clusion of Guttman and Weigt (1989) was morphological characteristics; that is, the that the rather small genetic distances esti- allelic products of specific genes in the pu- mated between the oak taxa (especially tative hybrid are either the same or differ- within subgenus Eryfhrobalanus) may re- ent from those of the suspected parental flect the extensive interspecific hybridiza- species. Despite this considerable advan- tion and introgression that has taken place tage, examination of Quercus spp by within the genus. Manos and Fairbrothers chemotaxonomic methods is restricted to a (1987) studied 6 species of the subgenus small number of reports and often these Erythrobalanus, and obtained similar re- are not specifically concerned with the oc- sults and thus concluded that, within the currence or incidence of hybridization but subgenus, evolution may have taken place with variation between species (eg, Bella- by morphological divergence accompanied rosa et al, 1990) or genetic diversity within by little electrophoretically detectable ge- species (eg Yacine and Lumaret, 1988, netic differentiation. 1989). Several reports are mentioned here A similar conclusion was reached by to focus attention on their potential or be- Chechowitz et al (1990) who showed that cause their results relate to hybridization oak populations in South Dakota and Wyo- within the genus generally. ming could not be distinguished electro- Perhaps the most wide-ranging enzyme phoretically from Q macrocarpa, whilst study was that of Santamour (1983) who morphologically they showed extensive in- surveyed cambial peroxidase isoenzymes trogression between Q macrocarpa and Q in over 90 taxa. Major bands enabled sub- that such gambelii. They argued disparity generic differences to be highlighted, but reflected natural selection operating differ- there was variability within the subgenera ently on morphological and electrophoretic Quercus, Cyclobalanus and Erythrobala- characters. Some species (eg Q rubra; nus so that individual species could not be Houston, 1983) have been shown to pos- sess extensive isoenzyme variation, which assigned to these subgenera on the basis would also preclude the use of isoenzymes of isoenzyme patterns alone. There was
in studies of their hybrids, whilst other mor- brids in the survey yielded somewhat dif- phologically very similar species can be re- ferent results. Q x bebbiana, thought to be solved electrophoretically (eg Q ilex, Q ro- a hybrid between Q alba and Q macrocar- tundifolia; Afzal-Rafii, 1988). Isozyme pa, was shown to possess a largely addi- variation has also been used to show con- tive phenolic profile. The parentage of Q siderable gene flow between island popu- comptonae was thought to be Q lyrata x Q lations of red oaks (Hokanson et al, 1991). virginiana, but chromatographically it was generally very similar to that of Q virginia- Discrepancy between morphological na and lacked the most prominent spot of and chemical characters has been inter- Q lyrata. preted very differently by Cristofolini (1985), who assessed seed and leaf pro- One of the most elegant studies of oak teins of a number of species. Whilst each hybridization using leaf phenols was that of species gave characteristic protein pat- Knops and Jensen (1980) involving 3 spe- terns, morphologically intermediate plants cies, Q ilicifolia, Q marilandica and Q velu- nearly always produced protein patterns tina; morphological data indicated that hy- corresponding to one or other of the sus- bridization was restricted to Q ilicifolia x Q pected parental species. The conclusion marilandica and Q marilandica x Q veluti- drawn was that morphological variation na. The 3 species had distinctive phenol may be due to phenotypic plasticity rather patterns which allowed detection and con- than to hybridization. An earlier investiga- firmation of the putative hybrid parentages tion of some of the same species (Olsson, and confirmed the lack of hybrids between 1975b) indicated that leaf peroxidase iso- Q ilicifolia and Q velutina. Cottam et al zymes showed high interspecific variation (1982) also used anthocyanidins and cate- in Q robur and Q petraea and that intro- chins to confirm the status of artificially gressed populations had affinities more raised hybrids. with Q petraea. Like isozymes, phenol can, however, be Because of the conflicting results al- variable within species and this variation ready shown by isoenzyme studies, it is may be related to the environment. unlikely that isoenzyme investigations will McDougal and Parks (1984) showed that generally provide accurate estimates of foliar phenols of Q rubra varied with eleva- the levels of natural hybridization, though tion (fig 3) and it was subsequently shown they may be useful in establishing individu- (McDougal and Parks, 1986) that the dif- al cases of hybridization. ferences in phenols between different ele- vations was largely under genetic control. As argued above for isoenzyme studies, Variation in phenolic compounds it is also unlikely that the use of phenolic compounds will prove particularly useful in An extensive study of phenols of American estimating levels of natural hybridization in oaks by Li and Hsaio (eg 1973) provided oak populations. the basis for our knowledge of phenolic variation in Quercus. Leaf phenols of 49 species were studied and, generally, the DNA phenolic pattern allowed differentiation of the subgenera Quercus, Protobalanus and Organellar DNAs show a high degree of Erythrobalanus, though the authors indi- potential for assessing levels of hybridity in cated that no one chromatographic spot was diagnostic for any subgenus. Two hy- natural populations (Whittemore and
Tucker (1961) showed that Q turbinella use in Quer- Schaal, 1991), though their and Q gambelii are ecologically separated, has so far been very limited. Whitte- cus the former living in semi-arid areas com- more and Schaal (1991) used variation of pared to the more mesic habitats at higher chloroplast DNA and nuclear ribosomal altitude of the latter, but where they are DNA extracted from winter buds and sympatric, in certain forest types, they hy- expanding leaves of 5 species of Ameri- bridize. Neilson and Wullstein (1985) re- can white oak which differed in many ported that the differential drought re- morphological characters but which had sponse of the 2 species is primarily due to different but overlapping geographical anatomical/morphological leaf differences ranges and ecological tolerances. Using but, unfortunately, no hybrids were stud- obviously non-hybrid individuals (as- ied. Rushton (1979) demonstrated that sessed on morphological data), they were populations, which were composed largely able to conclude that there were "... sev- of hybrids between Q robur and Q petraea eral clear cases of localized gene ex- and which may have been hybrid swarms, change between species, showing that were found along coastal river valleys in there is appreciable gene flow between Wales where they occupied intermediate sympatric species in this group." The rec- habitats between the better drained, sili- ognition that gene flow occurs in this ceous, nutrient-poor hill-tops and the wet- group without apparent morphological in- ter, more poorly drained nutrient-richer val- termediacy suggests that hybridization ley floors. may be more common than other data types indicate, and there is little doubt Disturbance may also be a factor in pro- that further investigations along these moting hybridization. For example, Silliman lines will assist in assessing levels of and Leisner (1958) showed that a mixed gene flow in Quercus. population of Q alba and Q montana on a stable, undisturbed site had no hybrids, whilst hybrids were common on a site sub- HYBRIDITY RELATED ject to successive disturbance by fire and TO THE HABITAT forestry. In an F generation, segregation and re- 2 The importance of the habitat in controlling combination would suggest that the individ- natural hybridization has been examined uals should be more heterogeneous in by Anderson (1948). He argued that the F their habitat requirements compared to ei- 1 hybrid should be uniform in its habitat re- ther the F generation or the original pa- 1 quirements and that these are likely to be rental species. Conversely, backcrossed intermediate between those of the 2 pa- F trees might be expected to show habitat 2 rental species (the ’hybrid habitat’). For ex- preferences similar to those of the back- ample, edaphic restriction of hybridization crossed parental species (Grant, 1971; is seen in Q harvardii, which is restricted Rushton, 1979). Benson et al (1967) have to deep, coarse sands, and Q mohriana, provided a clear example of such ecologi- which occurs on exposed limestone. cal segregation, though the method of data Where erosion creates a mixture of sand collection may be suspect. Hybrid popula- tions between Q douglassii and Q turbinel- and limestone fragments, hybrids are com- la subsp californica were examined and mon (Muller, 1952). In other species, it the composition of each population related may be climate which restricts hybridiza- to the degree of site exposure; on south- tion, as is the case of Q harvardii and Q west facing slopes, the populations were stellata (Muller, 1952).
successful intersubgeneric crosses. Jova- more like Q turbinella subsp californica, while Q douglasii-like novic et al (1973) also reported some suc- populations were cess in attempted hybridization between Q north-east facing prominent more on robur, Q alba, Q pubescens and Q pedun- slopes. Thus selection among the F gen- 2 eration for different recombinant types had culiflora. Whilst most interspecific crosses gave very low success rates (usually less occurred as a result of exposure differen- than ≈ 1.5%) compared with intraspecific ces. Evolutionary sorting in this instance crosses, the Q pedunculiflora x Q robur must be very rapid (Benson et al, 1967). cross was highly successful (30.7%). How- Consequently, the successful outcome ever, Cottam et al (1982) have viewed of a hybridization event in Quercus de- much of the work done in eastern Europe pends crucially upon the habitat condi- as dubious: "Most American tree geneti- tions, and the level of hybridity reported cists have tended to be sceptical about the under field conditions may be a reflection work done in eastern Europe." Wright more of the habitat restriction on establish- (1976) stated that "... the authenticity of ment than other factors. some is in doubt because the ’hybrids’ re- sembled the female parent only". This criti- cism is mild compared to some opinions ARTIFICIAL HYBRIDIZATION and comments made (not for publication) at geneticists’ gatherings. There have been 2 major research pro- The work of Cottam et al (1982) is well grammes aimed at producing artificial hy- documented and, in cases of doubt regard- brids, one led by Piatnitsky, started in 1937 ing the suspected hybrid, supplementary in Russia, and the other reported by Cot- data of F segregation, phenolic com- 2 tam et al (1982). The work of Piatnitsky pounds and epidermal characters (as seen was summarized in Piatnitsky (1960). In under the scanning electron microscope) all, over 200 000 pollinations were made were all utilized. The only data not provid- representing 47 different interspecific ed are the absolute success rates for each crosses, and 24 of these from 9 species cross made - the number of acorns and were considered successful (it should be subsequent seedlings are reported from noted, however, that Q fastigiata was con- the number of pollination ’sacks’ but no in- sidered a separate species rather than a dication is given of the number of female variety of Q robur). Many of the successful flowers in each sack. Nevertheless, the crosses were between species in the sub- programme was successful (table I) and genus Quercus but there were a number of resulted in 43 hybrid combinations. Inter-
sembling the parental species. Two differ- estingly, data given by Cottam et al (1982) for another programme (Schreiner, 1962) ent approaches have been used: 1) F 2 show an almost complete absence of suc- segregation to determine the parental spe- cess (table I), resulting, according to Cot- cies of natural hybrids (eg Allard, 1949; tam et al (1982), from a different pollina- Tucker and Bogert, 1973); and 2) the ex- tion method that may have "overpollinated" amination of the inheritance patterns in ar- the stigmas. tificial hybrids (see fig 4; and Yarnell, 1933). Where F generations were raised In addition to these 2 major research 2 by Cottam et al (1982), there was a clear programmes, there are numerous reports segregation of parental characters and, po- of more limited crossing experiments (eg, Dengler, 1941; Gegel’skii, 1975; Rushton, tentially, this would appear to be a most 1977). One general point to emerge from useful tool in establishing parentage (see of these studies is that certain spe- also Burk, 1965). some cies show a degree of self-incompatibility. Artificial resynthesis has therefore been achieved in a number of cases. However, LEVELS OF HYBRIDIZATION the lack of success in some instances IN NATURAL POPULATIONS where extensive natural hybridization has been reported (eg Q robur and Q petraea) It is not possible to generalize on the levels remains for further investigation. It should of hybridization in natural populations; in- be recalled that the inability to resynthe- stances are recorded of individual hybrid size hybrids artificially does not in itself in- trees (Tucker and Boger, 1973), barely in- validate the case for natural hybridization trogressed (eg Dupouey, 1983) to highly events. One other line of investigation introgressed populations (eg Cousens, would be to examine the frequency of oc- 1963) and hybrid swarms (Benson et al, currence of hybrids in natural populations 1967). What is apparent is that, in some of those species shown to have a high po- tential for successful artificial hybrid pro- species pairs that have been investigated over a wide geographical range, the levels duction. of hybridity vary considerably (see table II) though some of the differences could be KARYOTYPE ANALYSIS due to different methodologies of investiga- tion. Karyotype analysis has yielded little infor- (1965) believed that there Cousens was mation that can be of value in hybridization be- increasing cline of introgression an studies; the chromosomes are very small tween Q robur and Q petraea northwards and, although differences between species in England and Scotland. Rushton (1979) have been shown (eg Rushton, 1974; has shown a similar cline of increasing in- Wang, 1986), the karyotype is very uni- trogression from east to west in England form in those species examined. and Wales. The presence of suitable habi- tats for hybrid and backcross establish- ment may be a factor here, and planting of SEGREGATION 2 F important amenity and forestry species like these may have accelerated hybridization in some areas. In Scotland, the shorter Theoretically, the F should show a range 2 growing season may also allow a greater of types including the recovery of forms re-
CONCLUSION overlap of flowering period which would enhance hybrid formation. The extensive occurrence of hybrids with- It should be clearly emphasized, howev- in the genus is now well documented; er, that the view of widespread hybridiza- Hardin (1975) has observed that gene ex- tion in Quercus is not shared by all re- change "occurs or at least has the poten- searchers (eg, Jones, 1959), who have tial for taking place among nearly all spe- indicated that oaks are very plastic and cies of subgenus Quercus in eastern that the great range of variation in (largely) North America (albeit to a very limited ex- foliage characters with tree age, crown po- tent in most cases), and the species can sition, growing conditions etc, results in be thought of as comprising the most in- misinterpretations of patterns of variation. clusive breeding group or syngameon." To some extent this view is supported by This has led some authorities, such as the low percentage of success rates in Burger (1975), to question the traditional many artificial hybridization studies com- species concept within Quercus and for to intraspecific crosses (eg Rushton, pared some to suggest alternatives, such as the 1977; Ostrolucka, personal communica- multispecies (Van Valen, 1976), a con- tion); whilst it has been possible to pro- cept very like that of the syngameon duce numerous artificial hybrids, the actual (Grant, 1971).Itmust be recalled, howev- percentage of success rates is usually ex- recorded large proportion of er, that hy- tremely low. a
Bellarosa R, Delre V, Schirone B, Maggini F brids have been determined on morpho- (1990) Ribosomal RNA genes in Quercus spp logical grounds alone and that the per- (Fagaceae). Plant Syst Evol 172, 127-139 centage of success rates for artificially L, Phillips EA, Wilder PA, et al (1967) Benson raised hybrids is usually quite low. Re- Evolutionary sorting of characters in a hybrid evaluation of the systematic organization swarm I. Direction of slope. Am J Bot 54, of the genus in terms of species relation- 1017-1026 ships must await the more extensive ap- Blue MP, Jensen RJ (1988) Positional and sea- plication of alternative lines of evidence. sonal variation in oak (Quercus; Fagaceae) As Whittemore and Schaal (1991) state: leaf morphology. Am J Bot 75, 939-947 "Sympatric oak species are able to remain be- DR Brophy WB, Parnell (1974) Hybridization distinct despite considerable introgres- Quercus agrifolia and Q wislizenii tween sion, so that species concepts that rely on (Fagaceae). Madroño 22, 290-302 total genetic isolation between species to in Quer- Burger WC (1975) The species concept explain their distinctness clearly are not Taxon 24, 45-50 cus. applicable in Quercus." It will only be by Burk CJ (1965) Segregation in three genera- the application of a wide range of different tions of progeny of the hybrid oak Quercus x techniques that the levels of gene flow be- rudkini (Q marilandica x Q phellos). J Elisha tween Quercus species and the limits of Mitchell Sci Soc 81, 50-53 individual species will be accurately as- Carter AM (1974) Evidence for the hybrid origin sessed. of Cercidium sonorae (Leguminosae: Caesal- piniodeae) of northwestern Mexico. Madroño 22, 266-272 ACKNOWLEDGMENTS Carter AM, Rem NC (1974) Pollen studies in relation to hybridization in Cercidium and Parkinsonia (Leguminosae: Caesalpinioi- The author wishes to thank MC Lewis for first in- deae) of northwestern Mexico. Madroño 22, troducing him to Quercus hybrids over 20 years 303-311 ago and the staff of the Drawing and Photo- Chechowitz N, Chappell DM, Guttman SI, Weigt graphic Office, University of Ulster, for their help LA (1990) Morphological, electrophoretic, and in preparing diagrams for this paper. ecological analysis of Quercus macrocarpa populations in the Black Hills of South Dakota and Wyoming. Can J Bot 68, 2185-2194 REFERENCES Cottam WP, Tucker JM, Santamour FS (1982) Oak Hybridization at the University of Utah. State Arboretum of Utah Publ no 1, Salt Afzal-Rafii Z (1988) Caractéristiques taxono- Lake City, UT mique, morphologique et isoenzymatique du complexe "Chêne vert". Bull Soc Bot Fr Lett Cousens JE (1963) Variation of some diagnostic Bot 135, 343-352 characters of the sessile and pedunculate oaks and their hybrids in Scotland. Watsonia Allard HA (1949) An analysis of seedling proge- 5, 273-286 ny of an individual of Quercus saulii com- pared with seedlings of a typical individual of Cousens JE (1965) The status of the peduncu- the white oak (Quercus alba) and the typical late and sessile oaks in Britain. Watsonia 6, rock chestnut oak (Quercus montana). Cas- 161-176 tanea 14, 109-117 (1985) Electrophoretic data and Crawford DJ of the habitat. plant speciation. Syst Bot 10, 405-416 Anderson E (1948) Hybridization Evolution 2, 1-9 Cristofolini G (1985) Morphological and bio- chemical variability in European deciduous hybridization. Anderson E (1953) Introgressive oaks. Am J Bot 72, 931 Biol Rev 28, 280-307
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