Nuclear ribosomal DNA restriction-site variation in Penstemon section Peltanthera (Scrophulariaceae): an evaluation of diploid hybrid speciation and evidence for introgression

Penstemon spectabilis is a putative stabilized diploid hybrid of P. centranthifolius and P. grinnellii. It is morphologically intermediate, and all three species have different pollinators. Penstemon centranthifolius and P. spectabilis have been proposed as parents of P. clevelandii, which is purportedly isolated by ecological factors. Although hybridization between the proposed parental species has been reported, hybrid swarm formation has not been observed and introgression is purported to be minimal. We tested hypotheses of diploid hybrid speciation and introgression among these species based on rDNA restrictionsite and length variation for 56 populations within and outside of the hybrid complex. Unambiguous molecular markers clearly differentiated P. centranthifolius, P. grinnellii, and P. spectabilis, whereas P. spectabilis and P. clevelandii had the same rDNA type. The P. centranthifolius rDNA type was found in some populations of P. spectabilis and P. clevelandii, but there was no evidence of the additive profile documented for other recent diploid hybrid species. In contrast, the rDNA profile of P. × parishii had a completely additive profile of its proposed parental species P. centranthifolius and P. spectabilis. Ribosomal DNA markers for P. grinnellii were restricted to populations within the species and were not found in any population of P. spectabilis. Our data did not support hybrid-speciation hypotheses, but were in accord with allozyme data that provided evidence for introgression between P. centranthifolius and P. grinnellii, P. spectabilis and P. clevelandii. These results were used to propose criteria to differentiate ancient diploid hybrid speciation from patterns of introgression.     

Assessing hybridization in natural populations of Penstemon (Scrophulariaceae) using hypervariable intersimple sequence repeat (ISSR) bands

Inferences regarding hybridization rely on genetic markers to differentiate parental taxa from one another. Intersimple sequence repeat (ISSR) markers are based on single-primer PCR reactions where the primer sequence is derived from di- and trinucleotide repeats. These markers have successfully been used to assay genetic variability among cultivated plants, but have not yet been tested in natural populations. We used genetic markers generated from eight ISSR primers to examine patterns of hybridization and purported examples of hybrid speciation in Penstemon (Scrophulariaceae) in a hybrid complex involving P. centranthifolius , P. grinnellii , P. spectabilis and P. clevelandii . This hybrid complex has previously been studied using three molecular data sets (allozymes, and restriction-site variation of nuclear rDNA and chloroplast DNA). These studies revealed patterns of introgression involving P. centranthifolius , but were unsuccessful in determining whether gene flow occurs among the other species, and support for hypotheses of diploid hybrid speciation was also lacking. In this study, we were able to fingerprint each DNA accession sampled with one to three ISSR primers and most accessions could be identified with a single primer. We found population- and species-specific markers for each taxon surveyed. Our results: (i) do not support the hybrid origin of P. spectabilis ; (ii) do support the hypothesis that P. clevelandii is a diploid hybrid species derived from P. centranthifolius and P. spectabilis ; and (iii) demonstrate that pollen-mediated gene flow via hummingbird vectors is prevalent in the hybrid complex.     

Potential chromosomal introgression barriers revealed by linkage analysis in a hybrid of <Emphasis Type="Italic">Pinus massoniana</Emphasis> and <Emphasis Type="Italic">P. hwangshanensis</Emphasis>

Background  

Exploring the genetic mechanisms underlying speciation is a hot topic in modern genetics and evolutionary studies. Distortion of marker transmission ratio is frequently ascribed to selection against alleles that cause hybrid incompatibility. The natural introgression between P. massoniana and P. hwangshanensis and their distribution ranges lead to the emergence of the two species as desirable organisms to study the genetic mechanisms for speciation.     

Hybrid speciation in sparrows II: a role for sex chromosomes?

Homoploid hybrid speciation in animals is poorly understood, mainly because of the scarcity of well‐documented cases. Here, we present the results of a multilocus sequence analysis on the house sparrow (Passer domesticus), Spanish sparrow (P. hispaniolensis) and their proposed hybrid descendant, the Italian sparrow (P. italiae). The Italian sparrow is shown to be genetically intermediate between the house sparrow and Spanish sparrow, exhibiting genealogical discordance and a mosaic pattern of alleles derived from either of the putative parental species. The average variation on the Z chromosome was significantly reduced compared with autosomal variation in the putative parental species, the house sparrow and Spanish sparrow. Additionally, divergence between the two species was elevated on the Z chromosome relative to the autosomes. This pattern of variation and divergence is consistent with reduced introgression of Z‐linked genes and/or a faster‐Z effect (increased rate of adaptive divergence on the Z). F_ST‐outlier tests were consistent with the faster‐Z hypothesis: two of five Z‐linked loci (CHD1Z and PLAA) were identified as candidates for being subject to positive, divergent selection in the putative parental species. Interestingly, the two latter genes showed a mosaic pattern in the (hybrid) Italian sparrow; that is, the Italian sparrow was found to be fixed for Spanish sparrow alleles at CHD1Z and to mainly have house sparrow alleles at PLAA. Preliminary evidence presented in this study thus suggests that sex chromosomes may play a significant role in this case of homoploid hybrid speciation.     

Evolutionary history of Purple cone spruce (Picea purpurea) in the Qinghai–Tibet Plateau: homoploid hybrid origin and Pleistocene expansion

Hybridization and introgression can play an important role in speciation. Here, we examine their roles in the origin and evolution of Picea purpurea, a diploid spruce species occurring on the Qinghai–Tibet Plateau (QTP). Phylogenetic relationships and ecological differences between this species and its relatives, P. schrenkiana, P. likiangensis and P. wilsonii, are unclear. To clarify them, we surveyed sequence variation within and between them for 11 nuclear loci, three chloroplast (cp) and two mitochondrial (mt) DNA fragments, and examined their ecological requirements using ecological niche modelling. Initial analyses based on 11 nuclear loci rejected a close relationship between P. schrenkiana and P. purpurea. BP&P tests and ecological niche modelling indicated substantial divergence between the remaining three species and supported the species status of P. purpurea, which contained many private alleles as expected for a well‐established species. Sequence variation for cpDNA and mtDNA suggested a close relationship between P. purpurea and P. wilsonii, while variation at the nuclear se1364 gene suggested P. purpurea was more closely related to P. likiangensis. Analyses of genetic divergence, Bayesian clustering and model comparison using approximate Bayesian computation (ABC) of nuclear (nr) DNA variation all supported the hypothesis that P. purpurea originated by homoploid hybrid speciation from P. wilsonii and P. likiangensis. The ABC analysis dated the origin of P. purpurea at the Pleistocene, and the estimated hybrid parameter indicated that 69% of its nuclear composition was contributed by P. likiangensis and 31% by P. wilsonii. Our results further suggested that during or immediately following its formation, P. purpurea was subject to organelle DNA introgression from P. wilsonii such that it came to possess both mtDNA and cpDNA of P. wilsonii. The estimated parameters indicated that following its origin, P. purpurea underwent an expansion during/after the largest Pleistocene glaciation recorded for the QTP.     

GENETIC DIVERGENCE IN GALVEZIA (SCROPHULARIACEAE): EVOLUTIONARY AND BIOGEOGRAPHIC RELATIONSHIPS AMONG SOUTH AMERICAN AND GALÁPAGOS SPECIES

Nineteen populations representing all species of Galvezia were examined for electrophoretic differentiation at 23 loci. Allozyme variation within G. leucantha is minimal, suggesting that a genetic bottleneck, possibly resulting from a single colonization event, accompanied establishment of Galvezia in the Galàpagos Islands less than 1.5 million years ago. South American species have higher levels of genetic polymorphism. Interspecific divergence patterns in Galvezia are characterized by many qualitative allelic differences, including one to six marker alleles for each species. Isozyme data support delimitation of four species. Geographic modes of speciation predominate. Allozyme data indicate that the Galàpagos endemic is not a recent derivative from a continental progenitor; rather, it is most likely the sister species to G. fruticosa or to a lineage of G. fruticosa and an undescribed species from southern Peru. It is hypothesized that the presence of six fixed, unique alleles in G. leucantha identifies a relictual genotype from an extinct South American progenitor. Biogeographic patterns and the presence of an allele shared between G. leucantha and two populations from the Sechura Desert indicate that northern Peru is the probable source region for the Galàpagos introduction. Estimates of elapsed divergence times and paleoecological data suggest that continental species diverged in the late Pliocene or during the Pleistocene as a result of geographic isolation caused by climatic and vegetational change.     

Molecular Characterization of Parabasalian Symbionts Coronympha clevelandii and Trichonympha subquasilla from the Hawaiian Lowland Tree Termite Incisitermes immigrans

An important and undervalued challenge in characterizing symbiotic protists is the accurate identification of their host species. Here, we use DNA barcoding to resolve one confusing case involving parabasalian symbionts in the hindgut of the Hawaiian lowland tree termite, Incisitermes immigrans, which is host to several parabasalians, including the type species for the genus Coronympha, C. clevelandii. We collected I. immigrans from its type locality (Hawaii), confirmed its identity by DNA barcoding, and characterized the phylogenetic position of two symbionts, C. clevelandii and Trichonympha subquasilla. These data show that previous molecular surveys of “I. immigrans” are, in fact, mainly derived from the Caribbean termite I. schwarzi, and perhaps also another related species. These results emphasize the need for host barcoding, clarify the relationship between morphologically distinct Coronympha species, and also suggest some interesting distribution patterns of nonendemic termite species and their symbionts.     

Fine-scale genetic mapping of a hybrid sterility factor between <Emphasis Type="Italic">Drosophila simulans</Emphasis> and <Emphasis Type="Italic">D. mauritiana</Emphasis>: the varied and elusive functions of "speciation genes"

Background  

Hybrid male sterility (HMS) is a usual outcome of hybridization between closely related animal species. It arises because interactions between alleles that are functional within one species may be disrupted in hybrids. The identification of genes leading to hybrid sterility is of great interest for understanding the evolutionary process of speciation. In the current work we used marked P-element insertions as dominant markers to efficiently locate one genetic factor causing a severe reduction in fertility in hybrid males of Drosophila simulans and D. mauritiana.     

Needle morphological evidence of the homoploid hybrid origin of Pinus densata based on analysis of artificial hybrids and the putative parents,Pinus tabuliformis and Pinus yunnanensis

Genetic analyses indicate that Pinus densata is a natural homoploid hybrid originating from Pinus tabuliformis and Pinus yunnanensis. Needle morphological and anatomical features show relative species stability and can be used to identify coniferous species. Comparative analyses of these needle characteristics and phenotypic differences between the artificial hybrids, P. densata, and parental species can be used to determine the genetic and phenotypic evolutionary consequences of natural hybridization. Twelve artificial hybrid families, the two parental species, and P. densata were seeded in a high‐altitude habitat in Linzhi, Tibet. The needles of artificial hybrids and the three pine species were collected, and 24 needle morphological and anatomical traits were analyzed. Based on these results, variations in 10 needle traits among artificial hybrid families and 22 traits among species and artificial hybrids were predicted and found to be under moderate genetic control. Nineteen needle traits in artificial hybrids were similar to those in P. densata and between the two parental species, P. tabuliformis and P. yunnanensis. The ratio of plants with three needle clusters in artificial hybrids was 22.92%, which was very similar to P. densata. The eight needle traits (needle length, the mean number of stomata in sections 2 mm in length of the convex and flat sides of the needle, mean stomatal density, mesophyll/vascular bundle area ratio, mesophyll/resin canal area ratio, mesophyll/(resin canals and vascular bundles) area ratio, vascular bundle/resin canal area ratio) relative to physiological adaptability were similar to the artificial hybrids and P. densata. The similar needle features between the artificial hybrids and P. densata could be used to verify the homoploid hybrid origin of P. densata and helps to better understand of the hybridization roles in adaptation and speciation in plants.     

A molecular study of hybridization and homoploid hybrid speciation in Argyranthemum (Asteraceae) on Tenerife,the Canary Islands

Examples of recurrent homoploid hybrid speciation are few. One often‐cited example is Argyranthemum sundingii. This example includes two described species, A. lemsii and A. sundingii, resulting from reciprocal hybridization between A. broussonetii and A. frutescens on Tenerife. The four species and artificial F₁ and F₂ hybrids have previously been investigated morphologically and cytologically. Here, we examine population differentiation based on amplified fragment length polymorphism to get a better understanding of the genetic relationships among the species and the extent of hybridization. We aim to investigate if there is molecular support for treating the hybrid species as one taxon. Seven parental and four hybrid species populations (149 individuals) were analysed and we scored 85 polymorphic markers. A few (2–5) were private to each species but variably present and mostly rare. Our principal coordinate, STRUCTURE and BAPS analyses and AMOVA resulted in a clear separation of the parental species. The hybrid species were genetically less divergent but not identical. Our data indicate that hybridization and introgression are common in all these species on Tenerife and support the hypothesis that homoploid hybrid speciation has occurred repeatedly. Intrinsic post‐zygotic barriers are notoriously weak in Argyranthemum and reproductive isolation and speciation result primarily from strong ecological selection. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 19–31.     

Paulownia taiwaniana,a hybrid betweenP. fortunei andP. kawakamii (Scrophulariaceae)

Paulownia taiwaniana, the widely cultivated, commercially important tree, has been suspected of being of hybrid origin at least since its original publication in 1975. Evidence in support of this thesis, derived from a number of different investigations, is presented in this paper.—Strong evidence comes from a controlled pollination study of the two supposed parental species,P. kawakamii andP. fortunei. F₁ seedlings, derived from reciprocal crosses between the suspected parents, exhibited identical banding patterns for a number of enzymes (such as SKDH, GOT, and IDH) withP. taiwaniana, when separated by electrophoresis. Furthermore, comparative morphological studies of trichomes and wood parenchyma patterns between the purported parents andP. taiwaniana reveal that this latter qualitatively exhibits characteristics that combine features of both of the suspected parental types. Biochemically, eight enzyme systems were compared in the three species here under discussion, and, without exception, the electrophoretic banding patterns exhibited byP. taiwaniana represented a combination of the alleles of the other two species. Perhaps the most convincing evidence comes from a genetic analysis of the progeny obtained by selfingP. taiwaniana. Genotypic segregation of the offspring based on a single locus each of SKDH and PGI fit the 1:2:1 hypothesis. Genotypic segregation of the offspring based on two loci each of SP and GOT fit the ratio of 3:6:3:1:2:1. This, taken in conjunction with the other data presented, clearly suggests thatP. taiwaniana is a hybrid involvingP. kawakamii andP. fortunei.     

WEAK CROSSABILITY BARRIER BUT STRONG JUVENILE SELECTION SUPPORTS ECOLOGICAL SPECIATION OF THE HYBRID PINE PINUS DENSATA ON THE TIBETAN PLATEAU

Determining how a new hybrid lineage can achieve reproductive isolation is a key to understanding the process and mechanisms of homoploid hybrid speciation. Here, we evaluated the degree and nature of reproductive isolation between the ecologically successful hybrid species Pinus densata and its parental species P. tabuliformis and P. yunnanensis. We performed interspecific crosses among the three species to assess their crossability. We then conducted reciprocal transplantation experiments to evaluate their fitness differentiation, and to examine how natural populations representing different directions of introgression differ in adaptation. The crossing experiments revealed weak genetic barriers among the species. The transplantation trials showed manifest evidence of local adaptation as the three species all performed best in their native habitats. Pinus densata populations from the western edge of its distribution have evolved a strong local adaptation to the specific habitat in that range; populations representing different directions of introgressants with the two parental species all showed fitness disadvantages in this P. densata habitat. These observations illustrate that premating isolation through selection against immigrants from other habitat types or postzygotic isolation through selection against backcrosses between the three species is strong. Thus, ecological selection in combination with endogenous components and geographic isolation has likely played a significant role in the speciation of P. densata.     

ABRUPT CLINE FOR SEX CHROMOSOMES IN A HYBRID ZONE BETWEEN TWO SPECIES OF MICE

We compared the patterns of movement of sex chromosomal and autosomal loci along a 160 km transect across a zone of hybridization between M. domesticus and M. musculus in southern Germany and western Austria using seven genetic markers. These included one Y-specific DNA sequence (YB10), two X-specific loci (DXWas68 and DXWas31), and four autosomal isozyme loci (Es-10, Es-1, Mpi-1, and Np-1). Random effects logistic regression analysis enabled us to examine the relationship between M. domesticus allele frequency and geographic distance from the western edge of the hybrid zone and allowed statistical evaluation of differences in cline midpoint and width among loci. More limited movement was observed for all three sex chromosomal markers across the zone compared with three of the four autosomal markers. If differential movement reflects fitness differences of specific alleles (or alleles at closely linked loci) on a hybrid background, then alleles that move to a limited extent across a hybrid zone may contribute to hybrid breakdown between two species. The limited flow of both X- and Y-specific alleles suggest that sex chromosomes have played an important role in Mus speciation.     

Interspecific crosses between Tulipa gesneriana cultivars and wild Tulipa species: a survey

Summary Interspecific crosses were made between 76 Tulipa gesneriana cultivars and 28 botanical Tulipa species. Tulipa gesneriana proved fully congruent (all cross-combinations giving hybrid progeny) with members of the subsection Gesnerianae, was moderately affiliated (50% of all cross-combinations giving hybrid progeny) with representatives of the subsection Eichleres, and was found to be incongruent with most other Tulipa species (except T. stapfii of the subsection Oculussolis).Relationships between T. gesneriana and some representatives of the subsection Eichleres were addressed in more detail. Seeds were harvested in all cross-combinations, but the number of seeds depended on the T. gesneriana cultivar used as the female partner. Apparently, the maternal genotype did affect pre- or post-fertilization processes. T. gesneriana was not crossable with T. kaufmanniana of the subsection Spiranthera. In this specific case, however, a bridge cross was used to overcome incongruity. For this, T. kaufmanniana was first combined with T. greigii, and the hybrid was subsequently crossed with T. gesneriana.These results demonstrate possibilities for interspecific cross-combinations in tulips and have offered opportunities for the introduction of desired new traits in the cultivated tulip.     

ENZYME ELECTROPHORESIS AND EVOLUTIONARY RELATIONSHIPS AMONG THREE SPECIES OF LASTHENIA (ASTERACEAE: HELIANTHEAE)

Enzyme electrophoresis was used to examine variation at 18 gene loci in Lasthenia burkei, L. conjugens, and L. femontii. The species consist of diploid annuals restricted to vernal pools in California; a variety of data has indicated that they are closely related. Populations of the three species are similar at isozyme loci; L. conjugens and L. fremontii are most similar with a genetic identity of 0.965. Lasthenia conjugens and L. burkei are next most similar (0.934), while L. burkei and L. fremontii have an identity of 0.909. Lasthenia burkei and L. femontii each contain a different subset of the alleles found in L. conjugens. Electrophoretic data are concordant with the view that these three species are closely related, but do not support the hypothesis that L. burkei is a stabilized hybrid derivative of L. conjugens and L. fremontii because alleles unique to each of the latter two species are not combined in the former. Rather, distribution of alleles among the three species suggests two possible alternative hypotheses of evolutionary relationships. One hypothesis considers Lasthenia conjugens a hybrid derivative of L. burkei and L. fremontii while the alternative explanation views the latter two species as independent derivatives of the former. Neither of these hypotheses appears to be concordant with morphological data.     

Ecological speciation without host plant specialization; possible origins of a recently described cryptic Papilio species

North American Papilio canadensis and P. glaucus (Lepidoptera: Papilionidae, these Papilio = Pterourus) have previously been described as having allopatric distributions separated by a narrow hybrid zone running from Minnesota to southern New England, and southward in the Appalachian Mountains (possibly to northern Georgia). Recent patterns of hybridization and introgression suggest a more complex interaction between the two, possibly even resulting in the formation of a new species (Pterourus appalachiensis Pavulaan & Wright, 2002). Recently, extensive northward interspecific introgression of P. glaucus‐diagnostic traits has been observed in the hybrid zone. These include wing bands and other color patterns, the ability to feed on tulip tree leaves, and Hk‐100 allozymes; all are autosomally encoded. However, there has been little northward introgression of certain other P. glaucus traits (such as facultative diapause and bivoltinism, and Ldh‐100 allozymes, both X‐linked; and the Y‐linked melanic mimicry gene in females). Interspecific recombination of the X‐chromosome has evidently occurred, as shown by discordant patterns of X‐linked markers. The P. glaucus X‐linked Pgd‐100 and Pgd‐50 alleles have introgressed 200–400 km north of the historical hybrid zone, yet the P. glaucus X‐linked Ldh‐100 allele has not. The allele frequency shift for both genes is more closely related to the ‘thermal landscape’ (i.e., accumulated degree‐days above a developmental base threshold of 50 °F (=10 °C)) than to latitude. Delayed post‐diapause eclosion of cohorts within the hybrid zone, e.g., the New York/Vermont border area, has produced a natural ‘false‐second generation’ flight (a hybrid swarm of synchronous males and females, where 2300–2700 °F degree‐days have accumulated each year since 1998) that is reproductively isolated from flights of both parental species. Moreover, the newly described P. appalachiensis exhibits a unique combination of traits. These include obligate diapause, a univoltine habit, and the Ldh‐80 or Ldh‐40 alleles (as for P. canadensis), the Pgd‐100 or Pgd‐50 alleles (as for P. glaucus), and a delayed ‘false‐second generation’ reproductive flight period (as observed in the hybrid zone). Since 2001, a rare allele or ‘hybrizyme’ (Ldh‐20) has appeared in this false second generation at high frequencies (40–50%). We hypothesize that strong selection against the facultative diapause (od‐)trait (and the linked Ldh‐100 allele) in regions with 2800 °F degree‐days or less, and divergent selection in favor of Pgd‐100 (or a closely linked trait) combined with allochronic reproductive isolation, has resulted in recombinational, parapatric, hybrid speciation. There is no evidence at present that host‐plant shifts or changes in sex pheromones have driven this process, in contrast to many other speciation events in the Lepidoptera.     

Isolation and characterization of microsatellites in Primula glaucescens Moretti and their cross-species amplification in P. spectabilis Tratt.

The hexaploid herbaceous species Primula glaucescens Moretti and Primula spectabilis Tratt. (2n = 6x = 66) are two endemics of the Italian Pre-Alps protected by national and international laws. In order to plan conservation strategies for natural populations and to study the influence of the latest glaciation on them we developed a set of microsatellites markers for the endangered Primula species: ten primer pairs allowed analysis of polymorphic disomic loci in P. glaucescens samples and seven of them also amplified polymorphic disomic markers in P. spectabilis. Kirsten Wolff and Giorgio Binelli contributed equally to the paper     

Phylogeny of Saxifraga section Saxifraga subsection Arachnoideae (Saxifragaceae) and the origin of low elevation shade-dwelling species

Saxifraga section Saxifraga subsection Arachnoideae is a lineage of 12 species distributed mainly in the European Alps. It is unusual in terms of ecological diversification by containing both high elevation species from exposed alpine habitats and low elevation species from shady habitats such as overhanging rocks and cave entrances. Our aims are to explore which of these habitat types is ancestral, and to identify the possible drivers of this remarkable ecological diversification. Using a Hybseq DNA-sequencing approach and a complete species sample we reconstructed and dated the phylogeny of subsection Arachnoideae. Using Landolt indicator values, this phylogenetic tree was used for the reconstruction of the evolution of temperature, light and soil pH requirements in this lineage. Diversification of subsection Arachnoideae started in the late Pliocene and continued through the Pleistocene. Both diversification among and within clades was largely allopatric, and species from shady habitats with low light requirements are distributed in well-known refugia. We hypothesize that low light requirements evolved when species persisting in cold-stage refugia were forced into marginal habitats by more competitive warm-stage vegetation. While we do not claim that such competition resulted in speciation, it very likely resulted in adaptive evolution.     

Leaf anatomy of Ficus subsection Urostigma (Moraceae)

Species of Ficus subsection Urostigma show much overlapping variation in vegetative morphology, which often precludes correct identification of the species. The aim of this study was to describe the leaf anatomical characters and their variation and to check their suitability for identification. Included were 41 samples belonging to 25 species of subsection Urostigma, four samples belonging to two species of section Leucogyne and one specimen of Ficus glaberrima subsp. siamensis of subsection Conosycea. Transverse sections of lamina, midrib and petiole and cuticular macerations were used, and the observed anatomical characters are described for each species. On the basis of a limited number of studied samples, leaf anatomy shows little variation within each species and each species has a unique combination of character states, facilitating identification. Ficus arnottiana shows some leaf anatomical characters that are quite different from those of other members of subsection Urostigma, including a multiple epidermis and enlarged lithocysts on both sides of the leaf. Both characters are generally considered as typical for Ficus subsection Conosycea. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 259–281.     

HYBRIDIZATION LEADS TO SENSORY REPERTOIRE EXPANSION IN A GYNOGENETIC FISH,THE AMAZON MOLLY (POECILIA FORMOSA): A TEST OF THE HYBRID‐SENSORY EXPANSION HYPOTHESIS

Expansions in sensory systems usually require processes such as gene duplication and divergence, and thus evolve slowly. We evaluate a novel mechanism leading to rapid sensory repertoire expansion: hybrid‐sensory expansion (HSE). HSE occurs when two species with differently tuned sensory systems form a hybrid, bringing together alleles from each of the parental species. In one generation, a sensory repertoire is created that is the sum of the variance between parental species. The Amazon molly presents a unique opportunity to test the HSE hypothesis in a “frozen” hybrid. We compared opsin sequences of the Amazon molly, Poecilia formosa, to those of the parental species. Both parental species are homozygous at the RH2–1 locus and each of the four long wavelength sensitive loci, while P. formosa possess two different alleles at these loci; one matching each parental allele. Gene expression analysis showed P. formosa use the expanded opsin repertoire that was the result of HSE. Additionally, behavioral tests revealed P. formosa respond to colored stimuli in a manner similar or intermediate to the parental species P. mexicana and P. latipinna. Together these results strongly support the HSE hypothesis. Hybrid‐sensory repertoire expansion is likely important in other hybrid species and in other sensory systems.