Are the Homoploid Hybrid Species Ecologically Divergent from their Parents?
Although homoploid hybrid speciation is frequently proposed as an explanation for morphological intermediacy, there are still only a handful of instances in which these hypotheses have been rigorously confirmed using molecular markers (reviewed by Coyne and Orr 2004
; Rieseberg 1997
). The majority of these studies involve plants, but several examples have recently been reported from invertebrates and fish. In all cases, there is some measure of ecological divergence between the hybrid and its progenitors, although the thoroughness with which this has been documented varies. In most instances, the only information derives from brief descriptions of the habitat that hybrid neospecies occupies (see later discussion), although in few cases common garden and transplant experiments have been performed. Ecological divergence in a majority of known homoploid hybrid species would be unlikely if speciation was driven by intrinsic factors, such as chromosomal rearrangements. Overall, the consistent pattern of ecological divergence described conforms well to expectations if ecological divergence is important in promoting homoploid hybrid speciation ().
Confirmed and potential homoploid hybrid species, description of organism type, mode of ecological divergence from parental species, documentation of multiple origins, and references.
The first homoploid hybrid species to be confirmed with molecular markers was Stephanomeria diegensis
, a derivative of S. exigua
and S. virgata
(Gallez and Gottlieb 1982
). The hybrid species is found mainly in disturbed or pioneer habitats in southern California, a range that is restricted in both area and elevation when compared to those of its progenitors, which have extensive ranges in southwestern North America. S. exigua
is generally found in sandy soils occupied by sagebrush or creosote bush, whereas S. virgata
is common in chaparral openings and dry sandy hills.
also has a long history in the hybrid literature (e.g., Randolph 1966
), and its diploid hybrid ancestry was confirmed in 1993 (Arnold)
. There are three parental species all found in Louisiana: I. fulva
inhabits the shady, shallow water of bayou margins; I. hexagona
is found in sunny, deeper swamp water; and I. brevicaulis
occurs in much drier pastures and forests. In this system, the hybrid species occupies a divergent habitat that combines features from habitats of the parental species, and is found in the shady, deep water of cypress swamps.
Several hybrid species have been identified in the widespread Peaonia
complex of Europe and Asia (Ferguson and Sang 2001
; Sang et al. 1995
; Sang and Zhang 1999
). However, information on the ecology of the hybrid and parental Peaonia
species is not generally reported. Biogeographic distributions are known for many of the species, and the hybrid derivatives are often allopatric with respect to their progenitors. Interestingly, European populations of the progenitors appear to have been displaced by the hybrid species, perhaps due to the superior ability of the hybrids to adapt to Pleistocene climactic changes in Europe (Sang et al. 1995
The hybrid species Argyranthemum sundingii
(Brochmann et al. 2000
) exhibits a more classic kind of ecological divergence, occurring at an intermediate elevation and moisture gradient relative to the parents (Tenerife, Canary Islands). Greenhouse experiments have identified several phenotypic differences that are associated with habitat differentiation in the hybrid neospecies and shown that these differences are heritable.
The origin of Penstemon clevelandii
from P. centranthifolius
and P. spectabilis
is of interest because reproductive isolation of the hybrid neospecies may have been achieved by selection for a divergent pollination syndrome (Straw 1956
; Wolfe et al. 1998
). The hybrid species is bee- and hummingbird-pollinated, with magenta colored, semi-inflated flowers and occurs in canyons and road cuts in granitic soils of southern California and northern Baja. Penstemon spectabilis
is wasp-pollinated with lavender, fully inflated flowers and is a pioneer species of disturbed habitats in southern California and northern Baja, whereas P. centranthifolius
is hummingbird-pollinated with red, tubular flowers and is typically found in sandy washes and roadsides over the same general range as P. spectabilis
= 40) is unique relative to many other hybrid species; it has the same ploidy as the parental species S. vulgaris
= 40) but double the ploidy of the second parental species S. squalidus
= 20) (Abbott and Lowe 2004
). Thus it is neither a homoploid nor a polyploid hybrid species according to the strict definitions of these terms. Nonetheless, it is important to consider this example due to the fact that reproductive isolation between the S. eboracensis
and S. vulgaris
must be based on mechanisms other than ploidy changes. The hybrid species shows only slight ecological divergence relative to S. vulgaris
and the two taxa occur sympatrically in the British Isles, sometimes only meters away from each other. However, the species differ slightly in seed germination, seedling over wintering, flowering time, and floral morphology (Lowe and Abbott 2004
There are two genera in which diploid hybrid species clearly inhabit environments that are extreme relative to the parental species: Pinus
and Helianthus. Pinus densata
, which is derived from hybridization between P. yunnanensis
and P. tabulaeformis
, inhabits a high mountain environment where the parental species are apparently unable to grow (Wang et al. 2001
). The hybrid species occurs between 2700 and 4200 m above sea level on the Tibetan Plateau, whereas the parental species occur from sea level to 3100 m.
The genus Helianthus
(the North American sunflower) contains three hybrid species that occur in three widely divergent habitats (Heiser 1947
; Heiser et al. 1969
; Rogers et al. 1982
), despite sharing the same two parents () (Rieseberg 1991
). Both parental species are widely distributed across the United States; H. annuus
is found in mesic, clay-based soils and H. petiolaris
occurs in dryer, sandier soils. The hybrid species, in contrast, have restricted ranges in the southwestern United States; H. anomalus
is found on active sand dunes, H. deserticola
occupies xeric habitats in the Great Basin Desert, and H. paradoxus
inhabits desert salt marshes. Greenhouse comparisons have identified a suite of traits that differentiate each taxon from its parental species and that seem likely to confer a fitness advantage to the homoploid hybrid species in its native habitat (Rosenthal et al. 2002
; Schwarzbach et al. 2001
). For example, H. deserticola
has early flowering and small leaves compared to the parental species; both traits are typical of desert annuals. Transplant experiments indicate that the parental species fail to survive in H. paradoxus
habitat (Lexer et al. 2003b
) may be as fit as the hybrid taxon in the H. anomalus
habitat (Ludwig et al. in press
), and exhibit equivalent or greater fitness than the hybrid species in H. deserticola
habitat (Gross et al. 2004
). However, the parental species showed reduced or no emergence in the H. anomalus
habitat during two following experiments, suggesting that their exclusion may be due to failure in germination and early growth. Patterns of seedling emergence and survival from similar experiments in the H. deserticola
habitat remain unclear and will require further investigation (Ludwig and Donovan personal communication).
The best example of hybrid speciation in animals comes from the fish genus Gila
, native to the southwestern United States (DeMarais et al. 1992
). The parental species, G. robusta robusta
and G. elegans
, are found sympatrically in the large Colorado River, whereas the hybrid species, G. seminude
, is found only in the Virgin River, a moderately sized tributary of the Colorado. Interestingly, the parental species are never found in the Virgin River, although there are no known physical barriers that would impede their migration.
Homoploid hybrid speciation also has been proposed for the water flea Daphnia mendotae
, which occurs in slightly warmer water and shows less vertical migration than one if its sympatric progenitors, D. dentifera
; Taylor et al. 1996
). Hybrid speciation may also have occurred in the soft coral genus Alcyonium
, where the hybrid species is almost completely allopatric with its progenitors (McFadden and Hutchinson 2004
Does Ecological Divergence Contribute to Reproductive Isolation?
Ecological selection may contribute to reproductive isolation via habitat, floral/pollinator, and temporal divergence. All three types of divergence are potentially important in promoting homoploid hybrid speciation, but most research to date has focused on habiat divergence, which can lead to spatial isolation (a prezygotic barrier) and reduced fitness of hybrids in parental habitats (a postzygotic barrier). As noted by Coyne and Orr (2004)
, however, habitat divergence does not necessarily cause reproductive isolation. For example, plants might partition water usage in different ways but still flower simultaneously and hybridize readily. Do the ecological differences documented for various homoploid hybrid species actually contribute to reproductive isolation? Not surprisingly, this question remains to be investigated in many systems, but some answers are emerging.
Cases where hybrid species and their progenitors are partially or completely allopatric due to habitat divergence come the closest to satisfying the requirement of reproductive isolation. This group includes Peaonia
, and Gila
on a macrospatial scale and Stephanomeria
, and Argyranthemum
on a microspatial scale. Helianthus
is on the border between these extremes; the parental species overlap the hybrid taxa on a broad scale but generally occur several kilometers from each other at a given site. This spatial isolation appears to be an effective barrier to hybridization. For example, no hybridization has been detected between the ancient hybrid sunflower species and their progenitors, despite extensive fieldwork (Gross et al. 2003
; Heiser et al. 1969
; Rogers et al. 1982
; Schwarzbach and Rieseberg 2002
; Welch and Rieseberg 2002
). No attempts have been made to assess whether synthetic hybrids between a homoploid hybrid species and its progenitors would have reduced fitness in hybrid or parental habitats. However, the transplant experiments conducted for sunflowers (described earlier) provide a possible explanation for why the parental species fail to colonize the habitats of their hybrid derivatives.
Both temporal and pollinator divergence are thought to contribute to reproductive isolation between S. eboracensis
and S. vulgaris
. However, a thorough investigation of reproductive isolation in this system suggests that these ecological factors are probably less important in preventing hybridization than the fact that both species are predominantly self-fertilizing (Abbott and Lowe 2004
; Lowe and Abbott 2004
offers a possible example of hybrid speciation dependant on solely ecological isolating barriers (floral/pollinator isolation and possibly microspatial habitat isolation). The two parental species are pollinated by different animals, and the hybrid species is thought to exploit a third (Straw 1956
). Although anecdotally compelling, precise studies quantifying the degree of pollinator discrimination have not been conducted for the hybrid species, and hybridization between the derivative and parental species has been suggested (Chari and Wilson 2001
; Wilson and Valenzuela 2002
). Temporal divergence seems to be important in sunflower speciation, where Helianthus paradoxus
flowers substantially later than both parental species (Heiser et al. 1969
). However, the effectiveness of this temporal barrier to hybridization has not been quantified.
Can the New Combinations of Traits and Genes Responsible for Niche Divergence Be Generated via Hybridization?
One feature of homoploid hybrid speciation that renders it unique relative to most other modes of speciation is the potential for experimental manipulation. Synthetic hybrids between parental taxa can effectively serve as approximations of the ancestral genotype of the hybrid species, in contrast to the situation for most modern species, where the progenitor is unknown or extinct. It is straightforward, therefore, to test whether the traits or trait combinations required for ecological divergence could have been generated by hybridization. The alternative possibility is that the ecological divergence was mostly achieved after speciation through the gradual accumulation of new mutations. As far as we are aware, these experiments have only been carried in two systems: Argyranthemum and Helianthus.
Brochman et al. (2000) created synthetic F2 hybrids between Argyranthemum broussonetii and A. frutescens, the putative parents of the homoploid hybrid species A. sundingii, and grew them in the greenhouse along with the pure species. The hybrid species was intermediate between the two parental species for all nine measured traits, though usually closer to A. frutescens than A. broussonetii. The synthetic hybrids overlapped the phenotypic range of the homoploid hybrid species for eight of nine traits, showing that the traits could be produced easily via hybridization. The authors suggested that A. sundingii was likely generated via a backcross toward A. frutescens, so a BC1 or BC2 population would presumably have yielded an even greater overlap in phenotype.
In sunflowers, hybrid species exhibit many traits that are extreme relative to parental species, rather than being exclusively intermediate (Rosenthal et al. 2002
; Schwarzbach et al. 2001
). Multiple experiments have been conducted in the greenhouse and in the natural habitats of the three hybrid species to evaluate the potential for recreating these extreme traits in synthetic hybrids (second generation backcrosses toward both parents; hereafter BC2
Ann and BC2
Pet). Analyses of 40 traits in greenhouse-grown plants revealed that the ancient hybrid species’ trait values could be fully recovered in synthetic hybrids for all extreme traits in H. anomalus
and H. deserticola
and all but three extreme traits in H. paradoxus
(Rieseberg et al. 2003
; Rosenthal personal communication).
For the field experiments, which are more relevant ecologically, individuals of H. annuus
, H. petiolaris
, and one of the hybrid species were planted in that hybrid species’ native habitat along with the synthetic hybrids. However, the results were similar. Helianthus anomalus
, a desert sand dune endemic, was positively transgressive for leaf succulence and negatively transgressive for leaf nitrogen in its natural habitat (Ludwig et al. in press
). Synthetic hybrids overlapped H. anomalus
for both traits, although for leaf succulence the overlap was slight. Helianthus deserticola
, found in the xeric environment of the Great Basin Desert, was negatively transgressive for leaf area, stem diameter, and flowering date. As in the greenhouse studies, extreme values for all three traits were recovered in synthetic hybrids (Gross et al. 2004
). Helianthus paradoxus
inhabits highly saline desert marshes and field-grown plants were transgressive for five traits: sulfur, calcium, and boron content; leaf shape; and leaf succulence (Lexer et al. 2003b
). However, unlike the greenhouse experiments, synthetic hybrids overlapped the H. paradoxus
phenotype for all five traits despite the fact that only BC2
Pet hybrids were tested. These experiments show that extreme or transgressive phenotypes for ecologically relevant traits can be re-created via hybridization.
The sunflower work was extended to determine the genetic mechanism underlying the generation of extreme trait values in the synthetic hybrids and to ask if the same mechanism was responsible for production of similar traits during the origin of the ancient hybrid species more than 60,000 years ago (Gross et al. 2003
; Schwarzbach and Rieseberg 2002
; Welch and Rieseberg 2002
). Mapping of quantitative trait loci (QTLs) in the greenhouse-grown synthetic hybrids described earlier (Lexer et al. in press
; Rieseberg et al. 2003
), as well as those raised in the H. paradoxus
habitat (Lexer et al. 2003a
) revealed that extreme or transgressive trait values were almost certainly generated by complementary gene action. This makes sense given that complementary gene action has proven to be the primary cause of transgressive segregation in crop hybrids (e.g., deVincente and Tanksley 1993
). Furthermore, it was shown that closely linked or pleiotropic QTLs generally had effects in same direction with respect to the phenotypes of the hybrid species. Given these favorable genetic correlations, it should be feasible to generate the complex, multitrait phenotypes of the ancient hybrid species after only a few generations of recombination.
Despite evidence of feasibility, the possibility remains that phenotypic/ecological divergence was mostly achieved after speciation through the gradual accumulation of new mutations and that the detected hybridization was coincidental to the process. This objection is laid to rest, however, by comparisons of the QTL mapping data with detailed genetic maps of the diploid hybrid species (Rieseberg et al. 2003
). If the phenotypes of the diploid hybrid species arose through complementary gene action as hypothesized, then we should be able to predict the genomic composition of the hybrid species from the QTL mapping data. Parentage of 71.8% of markers in H. paradoxus
, 75.6% of markers in H. anomalus
, and 79.3% of markers in H. deserticola
was predicted correctly (p
.0001 for all comparisons), indicating that hybridization and complementary gene action did contribute to the phenotypic and ecological divergence of ancient sunflower hybrids.
Are Traits and Genes Responsible for Ecological Divergence under Divergent Natural Selection in the Habitat of the Hybrid Species?
Synthetic hybrids are useful not only for evaluating trait expression but also for re-creating the selective pressures acting on hybrid neospecies at their origin. If ecological selection was important in speciation and shaped the phenotype and genotype of the hybrid species, then selection acting on synthetic hybrids in the field should favor the trait values found in present-day populations of the ancient hybrid species. Such investigations have only been carried out in Helianthus to date; we report the results here.
Parallel phenotypic selection experiments (Lande and Arnold 1983
) have been carried out in the habitat of all three hybrid sunflower species to estimate the strength and direction ecological selection acting on early generation hybrids in the field. Selection differentials and gradients were calculated for synthetic BC2
hybrids (described earlier), with a focus on traits that are transgressive in the hybrid species today. Overall, these field experiments have shown that many of the extreme traits found in the ancient hybrid species could have arisen via divergent ecological selection acting on transgressive hybrids growing in novel habitats. These traits include high leaf succulence in H. anomalus
(Ludwig et al. in press
), small leaves and early flowering in H. deserticola
(Gross et al. 2004
), and high leaf succulence, high calcium content, and low uptake of toxic elements in H. paradoxus
(Lexer et al. 2003b
However, these studies failed to detect significant selection on several traits, and in several instances selection was not in the predicted direction. It may be that these contradictory results are an artifact of the restricted spatial and temporal scale of the selection experiments. Given that selection fluctuates over both time and space (Grant and Grant 2002
), it is not surprising that selective pressures measured over a single growing season are not fully consistent with predictions based on current phenotypes of the hybrid taxa. Despite these shortcomings, the experiments successfully demonstrate that divergent ecological selection may allow transgressive sunflower hybrids to colonize new habitats and thereby achieve some degree of habitat isolation from their progenitors.
Is Selection Strong Enough to Allow Divergence in the Face of Gene Flow?
Because hybrids are necessarily produced in proximity with one or both of their parental species, homoploid hybrid speciation represents a kind of speciation with gene flow, which requires that the strength of selection on individual loci, s
, exceed the migration rate, m
. The fact that introgression is a far more frequent outcome of hybridization than speciation implies either that this condition is rarely met or that open niches are rarely available for colonization by hybrids (Arnold 1997
; Buerkle et al. 2000
; Rieseberg and Wendel 1993
). In those cases where hybrid speciation has occurred, it is necessary to determine whether ecological selection alone would have been strong enough to drive divergence or whether migration rates had to be reduced by some other form of isolation (e.g., chromosomal or spatial isolation) before ecological divergence could occur.
The parameter m can be estimated fairly easily from population-level surveys of molecular markers, such as microsatellites. Approximating s is more challenging, because loci controlling a trait of interest must identified and relative fitness must be estimated for individuals that segregate for parental species’ alleles at that locus. This combination of fieldwork and genetic mapping is rare, so the relationship between migration and selection has only been calculated for hybrid taxa in the genus Helianthus.
The phenotypic selection study in H. paradoxus
was extended to the genetic level via QTL analyses of the population of synthetic hybrids grown in salt marsh habitat. Selection coefficients on individual salt tolerance QTLs in the H. paradoxus
habitat were large (+0.126, −0.084, and −0.094) (Lexer et al. 2003a
). Current estimates of the parameter Nem
for annual sunflowers are consistently less than or equal to 1. Thus, a slight excess of 12 individuals would be sufficient to allow selection on the founding population of the H. paradoxus
lineage to overcome the homogenizing effects of gene flow. The analysis also revealed that QTLs contributing to increased fitness in the salt marsh were derived from both parental species, as predicted by the complementary gene action model if hybridization facilitated adaptation to this new environment.
The phenotypic selection study on the desert floor was likewise expanded to the genetic level, focusing on the BC2
Pet cross due to its similarity to the H. deserticola
phenotype (Gross unpublished data). The study in the H. deserticola
habitat took advantage of the large number of QTLs controlling morphological, physiological, and life history traits that had been documented for the BC2
Pet cross in a previous greenhouse study (Rieseberg et al. 2003
), rather than constructing a new QTL map. Each chromosomal interval containing a QTL for a trait of interest in the greenhouse was considered as a candidate QTL region in the field population, and the BC2
Pet field population was genotyped for the microsatellite markers flanking these intervals. Microsatellites bounding the candidate intervals for each trait were compiled into map order based on Lexer et al. (2003a)
, and QTL presence was tested using interval mapping in Map Manager QTX (Manly et al. 2001
). Tests were performed at 1 cm steps, and threshold values for declaring the potential presence of a QTL were determined by 1,000 permutations of the data for each trait. Three QTLs were found; two that were significant at p
< .05 and one with marginal significance (p
= .065). The approximate location of a QTL was established as the chromosomal region where the likelihood ratio score (LRS) exceeded the significance level.
Selection coefficients on individual QTLs contributing to adaptive traits (leaf width and stem diameter) in the H. deserticola environment ranged from −0.027 to −0.050 (). Although these values are somewhat lower than those reported for the H. paradoxus habitat, they still are considerably larger than those likely required for divergence in parapatry. Our crude estimates indicate that divergence could occur so long as the ancestral population exceeded ≈ 37 individuals, which seems plausible given that most sunflower populations number in the hundreds or thousands.
Table 3 Putative QTL linkage groups, interval markers, positions, percent phenotypic variance explained (PVE), additive effects, proxy microsatellite marker, and selection coefficient (s) for two traits under selection in a second generation backcross (BC2) population (more ...)
Is There Evidence of Parallel Hybrid Speciation?
argues that recurrent speciation is unlikely in the absence of parallel selective pressures resulting from adaptation to similar features in the environment. Parallel speciation is therefore viewed as “compelling evidence that divergent natural selection has ultimately brought about the evolution of reproductive isolation” (Schluter 2000, p. 378
). However, there are possible exceptions to this general rule. Polyploid species, for example, frequently have multiple origins in the absence of parallel selection. Likewise, homoploid hybrid species may converge toward a similar genomic composition due to fertility selection rather than parallel natural selection (Rieseberg 2000
; Rieseberg et al. 1996
). However, in homoploid hybrid speciation, fertility selection does not typically confer parallel ecological changes (Rieseberg personal observations). Thus the recurrent formation of ecologically isolated homoploid hybrid species does indeed provide evidence that speciation was driven in part by divergent natural selection.
Have the homoploid hybrid species described earlier arisen multiple times? This is a difficult question to answer because patterns of molecular variation that appear most consistent with multiple hybrid speciation events almost always have alternative explanations (Gross et al. 2003
). For example, the common observation that populations of the homoploid hybrid species show geographic partitioning of molecular markers can be explained by multiple hybrid speciation events or a single speciation event, followed by the sorting of ancestral variation or local introgression between the hybrid species and one of the parental species. Therefore, the evidence supporting claims of multiple origins must be considered carefully.
was the first homoploid hybrid species for which multiple origins were proposed (Brochmann et al. 2000
). Recurrent speciation was inferred from the presence of different chloroplast DNA types (one from each parent), in the two known populations of the species. The authors suggested that each population had been generated in situ in separate valleys from local parental populations, although it is impossible to disprove that this pattern resulted from a single origin followed by dispersal. More recently, the researchers documented some karyotypic divergence in different populations of the hybrid species, providing further support for multiple origins (Borgen et al. 2003
Possible multiple origins of the diploid hybrid species P. densata
have been investigated carefully, taking advantage of the fact that chloroplast DNA is paternally inherited and mitochondrial DNA is maternally inherited in Pinus
(Song et al. 2003
; Wang et al. 2001
). Patterns of variation in allozymes, chloroplast DNA, and mitochondrial DNA show that different populations of P. desata
have very diverse genetic compositions, with varying degrees of genomic contributions from each parental species. Strikingly, each parental species has served as a maternal parent in some P. densata
populations and a paternal parent in others (i.e., the hybrid species shows reciprocal parentage). These differences suggest that populations of the hybrid species have experienced unique evolutionary histories and most likely have independent origins.
The repeatability of diploid hybrid speciation in H. anomalus and H. deserticola was evaluated using evidence from patterns of variation in chloroplast DNA, nuclear microsatellite loci, and population cross-viability/chromosomal structure. A single origin would likely result in a single chloroplast DNA type, monophyly for the hybrid species based on microsatellites, and high interfertility among all populations. Multiple origins, on the other hand, could potentially result in multiple chloroplast DNA types, paraphyly for the hybrid species, and perhaps sharp divisions in interfertility among populations.
was monophyletic based on microsatellite loci, but the patterns of chloroplast DNA variation and crossability were consistent with three independent origins of the homoploid hybrid species (Schwarzbach and Rieseberg 2002
). This combination of data is especially compelling because differences in chloroplast DNA type were correlated with population interfertility, a pattern that would be unlikely if variation in either trait were due to introgression with the parental species. Sorting of ancestral variation also seemed an unlikely explanation for the pattern, in that it would require that the ancestral population include chloroplast DNA from geographically distant populations of the parental populations. H. deserticola
was paraphyletic with the parental species H. petiolaris
based on microsatellites, and also contained chloroplast DNA types characteristic of both parental species (Gross et al. 2003
). Although these patterns are suggestive of multiple origins, they were not as well supported by population interfertility as in the H. anomalus
There are now four diploid hybrid species that are thought to be the result of parallel speciation. Even if the evidence in some of these cases is due to causes other than multiple origins, it is remarkable that this outcome is supported so much more frequently than single origins, which have only been confirmed once (Welch and Rieseberg 2002
). Given the limited number of homoploid hybrid species in the literature, the large proportion that appear to be multiply derived strongly implies that ecology is a frequent and significant contributor to this mode of speciation.