A search for Y-chromosomal species-specific markers and their use for hybridization analysis in ground squirrels

In four ground squirrel species from the Volga region-yellow (Spermophilus fulvus), russet (S. major), little (S. pygmaeus), and speckled (S. suslicus)--four hybridization variants (major/fulvus, major/pygmaeus, major/suslicus, and pygmaeus/suslicus) have been reliably described. Earlier we have shown that populations of S. major from the Volga region were characterized by wide introgression of mtDNA from S. fulvus and S. pygmaeus, which probably, resulted from ancient hybridization. In this study, the same populations were used to analyze the introgression of the Y chromosome, which (unlike mtDNA) is paternally inherited. Three genes, ZfY, SRY, and SmcY were tested as Y-chromosomal candidate markers. It was demonstrated that Y chromosome of ground squirrels lacked the ZfY gene, while its homologous structure, ZfY(X), was presumably linked to the X chromosome. The SRY region examined was rather conservative. In particular, the sequences determined in S. major and S. fulvus were identical, while three out of four substitutions found in S. pygmaeus were located in the coding region. The SmcY gene was found to be the most suitable marker, providing distinguishing of all of the four ground squirrel species by nine nucleotide substitutions. Introgression at the Y chromosome was observed only in two cases: in one S. major individual (out of 51 phenotypically pure animals) caught in the major/fulvus sympatry zone, and in four (one litter) out of fourteen S. fulvus individuals caught in close vicinity of the sympatry zone of these two species. Among 28 S. pymaeus and 9 S. suslicus individuals, no foreign SmcY genes were detected. Two colonies of the "hybrid accumulation" type were examined with eight major/suslicus hybrids analyzed in the first and seventeen major/fulvus hybrids in the second colony. The prevalence of the S. major paternal lineages was observed in both colonies (87.5 and 82.4%, respectively). The data obtained suggest that compared to wide mtDNA introgression, introgression of Y chromosome in the Volga region ground squirrels is statistically significantly less frequent event.     

A Molecular Genetic Study of Hybridization in Four Species of Ground Squirrels (Spermophilus: Rodentia,Sciuridae)

Four species of ground squirrel—yellow (Spermophilus fulvus), russet (S. major), small (S. pygmaeus), and spotted (S. suslicus)—occur in the Volga region. Between S. major and S. pigmaeus, S. major and S. fulvus, and S. major and S. suslicus, sporadic hybridization was reported. Using sequencing and restriction analysis, we have examined the mtDNA C region in 13 yellow, 60 russet, 61 small, 45 spotted ground squirrels, and 9 phenotypic hybrids between these species. It was shown that 43% of S. major individuals had alien mitotypes typical of S. fulvus and S. pygmaeus. Alien mitotypes occurred both within and outside sympatric zones. No alien mitotypes were found in 119 animals of the other three species, which suggests that only one parental species (S. major) predominantly participates in backcrosses. Phenotypic hybrids S. fulvus × S. major and S. major × S. pygmaeus) were reliably identified using RAPD–PCR of nuclear DNA. However, we could find no significant traces of hybridization in S. major with alien mitotypes. Analysis of p53 pseudogenes of S. major and S. fulvus that were for the first time described in the present study produced similar results: 59 out of 60 individuals of S. major (including S. major with S. fulvus mitotypes) had only the pseudogene variant specific for S. major. This situation is possible even at low hybridization frequencies (less than 1% according to field observations and 1.4 to 2.7% according to nuclear DNA analysis) if dispersal of S. major from the sympatric zones mainly involved animals that obtained alien mtDNA via backcrossing. The prevalence of animals with alien mitotypes in some S. major populations can be explained by the founder effect. Further studies based on large samples are required for clarifying the discrepancies between mitochondrial and nuclear DNA data.     

A search for Y-chromosomal species-specific markers and their use for hybridization analysis in ground squirrels (Spermophilus: Rodentia, sciuridae)

In four ground squirrel species from the Volga region—yellow (Spermophilus fulvus), russet (S. major), little (S. pygmaeus), and speckled (S. suslicus)—four hybridization variants (major/fulvus, major/pygmaeus, major/suslicus, and pygmaeus/suslicus) have been reliably described. Earlier we have shown that populations of S. major from the Volga region were characterized by wide introgression of mtDNA from S. fulvus and S. pygmaeus, which probably, resulted from ancient hybridization [5]. In this study, the same populations were used to analyze the introgression of the Y chromosome, which (unlike mtDNA) is paternally inherited. Three genes, ZfY, SRY, and SmcY were tested as Y-chromosomal candidate markers. It was demonstrated that Y chromosome of ground squirrels lacked the ZfY gene, while its homologous structure, ZfY(X), was presumably linked to the X chromosome. The SRY region examined was rather conservative. In particular, the sequences determined in S. major and S. fulvus were identical, while three out of four substitutions found in S. pygmaeus were located in the coding region. The SmcY gene was found to be the most suitable marker, providing distinguishing of all of the four ground squirrel species by nine nucleotide substitutions. Introgression at the Y chromosome was observed only in two cases: in one S. major individual (out of 51 phenotypically pure animals) caught in the major/fulvus sympatry zone, and in four (one litter) out of fourteen S. fulvus individuals caught in close vicinity of the sympatry zone of these two species. Among 28 S. pymaeus and 9 S. suslicus individuals, no foreign SmcY genes were detected. Two colonies of the “hybrid swarm” type were examined with eight major/suslicus hybrids analyzed in the first and seventeen major/fulvus hybrids in the second colony. The prevalence of the S. major paternal lineages was observed in both colonies (87.5 and 82.4%, respectively). The data obtained suggest that compared to wide mtDNA introgression, introgression of Y chromosome in the Volga region ground squirrels is statistically significantly less frequent event.     

Biotope principles of sympatry and interspecies hybridization in mammals (by the example of the genus <Emphasis Type="Italic">Spermophilus</Emphasis>)

Two hybridization zones were taken as an example (the russet ground squirrel (Spermophilus major) and the speckled ground squirrel (S. suslicus), the russet ground squirrel and the yellow ground squirrel S. fulvus) to show that biotope characteristics determine segregation of sympatric species in contact colonies. The heterogeneity of biotopes with regard to the requirements of sympatric species promotes their long-term and steady dispersal and, in rare cases, sporadic hybridization. A biotope with a homogenous environment enables wide interspecies hybridization and a rapid increase in the ratio of hybrids in the colony.     

Spatial partitioning and asymmetric hybridization among sympatric coastal steelhead trout (Oncorhynchus mykiss irideus), coastal cutthroat trout (O. clarki clarki) and interspecific hybrids

Hybridization between sympatric species provides unique opportunities to examine the contrast between mechanisms that promote hybridization and maintain species integrity. We surveyed hybridization between sympatric coastal steelhead (Oncorhynchus mykiss irideus) and coastal cutthroat trout (O. clarki clarki) from two streams in Washington State, Olsen Creek (256 individuals sampled) and Jansen Creek (431 individuals sampled), over a 3-year period. We applied 11 O. mykiss-specific nuclear markers, 11 O. c. clarki-specific nuclear markers and a mitochondrial DNA marker to assess spatial partitioning among species and hybrids and determine the directionality of hybridization. F1 and post-F1 hybrids, respectively, composed an average of 1.2% and 33.6% of the population sampled in Jansen Creek, and 5.9% and 30.4% of the population sampled in Olsen Creek. A modest level of habitat partitioning among species and hybrids was detected. Mitochondrial DNA analysis indicated that all F1 hybrids (15 from Olsen Creek and five from Jansen Creek) arose from matings between steelhead females and cutthroat males implicating a sneak spawning behaviour by cutthroat males. First-generation cutthroat backcrosses contained O. c. clarki mtDNA more often than expected suggesting natural selection against F1 hybrids. More hybrids were backcrossed toward cutthroat than steelhead and our results indicate recurrent hybridization within these creeks. Age analysis demonstrated that hybrids were between 1 and 4 years old. These results suggest that within sympatric salmonid hybrid zones, exogenous processes (environmentally dependent factors) help to maintain the distinction between parental types through reduced fitness of hybrids within parental environments while divergent natural selection promotes parental types through distinct adaptive advantages of parental phenotypes.     

Genetic evidence of extensive introgression of short-tailed ground squirrel genes in a hybridization zone of Spermophilus major and S. erythrogenys, inferred from sequencing of the mtDNA cytochrome b gene

We have completely sequenced the mtDNA cytochrome b gene of ground squirrels from the zone of overlapping ranges of Spermophilus major and S. erythrogenys in the Tobol-Ishim interfluve, which is a putative hybridization zone of these species. The results of the sequencing showed extensive introgression of mtDNA genes of the short-tailed ground squirrel S. e. brevicauda, whose haplotype had fully replaced the S. major haplotype. All of the ground squirrels from the Tobol-Ishim interfluve had a variant of the S. e. brevicauda mtDNA haplotype that was specific for this zone. On average, 119 substitutions (10.44%) were found between S. major from Ul'yanovsk oblast and S. e. brevicauda from the northern Kazakhstan, the mean genetic distance (D) between them being 0.115, which conforms to the corresponding parameters for the S. e. brevicauda-S. pygmaeus pair (122 substitutions, D = 118). Insignificant differences (seven substitutions, D = 0.043) were found between the S. major and S. pygmaeus haplotypes, which suggest that these species have similar mitochondrial haplotypes. Five to ten nucleotide substitutions (0.44--0.88%) were detected between the animals from the Tobol--Ishim interfluve and S. e. brevicauda. The mtDNA haplotype divergence D within the genus Spermophilus (ten species) for all codon positions ranged from 0.035 to 0.158. Phylogenetic reconstructions (MP, ML, and NJ trees) showed two well-differentiated clusters with high bootstrap support. However, there was different branching topology within the cluster and their species composition varied. The maximum likelihood tree, ML, differentiating the species into two subgenera, Citellus and Colobotis, most reliably reflected taxonomic relationships of the species from the genus Spermophilus, inferred from morphological and genetic biochemical data. The morphologically pure S. major (subgenus Colobotis) animals, used in the analysis, proved to carry the haplotype of another species, S. pygmaeus (subgenus Citellus). This poses a question on the existence of the specific haplotype of S. major, the reason of its replacement by haplotype of other species, and possible consequences of this phenomenon for survival of the species.     

The potential for hybridization in freshwater copepods

The importance of hybridization and hybrid zones in pelagic systems is largely unknown, in part because planktonic species are generally assumed to be reproductively isolated. However, lakes in their entirety represent potential hybrid zones throughout which sympatric species may mate and hybridize. Recent field evidence for the existence of intermediate phenotypes together with behavioral observations of heterospecific matings suggest that hybridization may be much more common than previously thought in these systems. This study examines the potential for hybridization of two related copepod species, Diaptomus minutus and D. pygmaeus, that co-occur in lakes throughout the northeastern United States. Field and experimental laboratory data were collected to: (1) compare the spatial and temporal occurrence of these two congeners in a single lake; (2) quantify the extent to which mating errors occur insitu; (3) examine the species specific mating cues that potentially affect the probability of hybridization; and (4) determine the potential for gametic compatibility and hybrid viability of these two species. D. minutus and D. pygmaeus are spatially and temporally sympatric, and the timing of their reproductive activity can be coincident insitu. One-directional mating errors occur frequently in both the laboratory and the field (e.g., upto 70% of D.minutus females in situ may carry spermatophores from D.pygmaeus males). Very low but successful production of hybrids also occurs, demonstrating that their gametes are indeed compatible and their hybrids are sometimes viable. These results underscore the potential for hybridization to play a greater role in speciation and contribute more to phenotypic diversity in aquatic crustacean communities than previously shown. Received: 25 March 1996 / Accepted: 4 April 1997     

Hybridization and Introgression among Species of Sunfish (Lepomis): Analysis by Mitochondrial DNA and Allozyme Markers

We explore the potential of mitochondrial DNA (mtDNA) analysis, alone and in conjunction with allozymes, to study low-frequency hybridization and introgression phenomena in natural populations. MtDNAs from small samples of nine species of sunfish (Lepomis, Centrarchidae) were purified and digested with each of 13 informative restriction enzymes. Digestion profiles for all species were highly distinct: estimates of overall fragment homology between pairs of species ranged from 0-36%. Allozymes encoded by nine nuclear genes also showed large freqency differences among species and together with mtDNA provided many genetic markers for hybrid identification. A genetic analysis of 277 sunfish from two locations in north Georgia revealed the following: (1) a low frequency of interspecific hybrids, all of which appeared to be F1's; (2) the involvement of five sympatric Lepomis species in the production of these hybrids; (3) no evidence for introgression between species in our study locales (although for rare hybridization, most later-generation backcrosses would not be reliably distinguished from parentals); (4) a tendency for hybridizations to take place preferentially between parental species differing greatly in abundance; (5) a tendency for the rare species in a hybrid cross to provide the female parent. Our data suggest that absence of conspecific pairing partners and mating stimuli for females of rarer species may be important factors in increasing the likelihood of interspecific hybridization. The maternal inheritance of mtDNA offers at least two novel advantages for hybridization analysis: (1) an opportunity to determine direction in hybrid crosses; and (2) due to the linkage among mtDNA markers, an increased potential to distinguish effects of introgression from symplesiomorphy or character convergence.     

Introgressive hybridization between Dolly Varden Salvelinus malma and white‐spotted charr Salvelinus leucomaenis on Hokkaido Island, Japan

Intra‐ and interspecific phylogenetic analysis of Dolly Varden Salvelinus malma and white‐spotted charr Salvelinus leucomaenis throughout Hokkaido Island was conducted using nucleotide sequences of the mitochondrial DNA control region and three nuclear DNA markers [ribosomal DNA (ITS‐1), microsatellite ( u ‐85), SINE ( Fok ‐223)]. A total of 19 mtDNA haplotypes from 271 individuals of Dolly Varden, nine haplotypes from 134 white‐spotted charr were defined, and identified two well‐supported monophyletic clades for each species. Two haplotypes of Dolly Varden, however, were clustered together in the white‐spotted charr clade. Those haplotypes were found only from several rivers in the Shiretoko Peninsula, and a neighbouring river of that region. Analyses of ribosomal DNA and Fok ‐223 loci revealed that both species are characterized by having completely species‐specific diagnostic sequence and fragment patterns. Dolly Varden of the Shiretoko populations are typically allopatric in distribution where white‐spotted charr do not currently occur. In addition, incongruence in genetic relationships between mtDNA and nuclear DNA markers give strong evidence of historical mtDNA introgression between Dolly Varden and white‐spotted charr. Some white‐spotted charr diagnostic alleles in a nuclear microsatellite locus ( u ‐85) were found in some Shiretoko Dolly Varden populations, suggesting that introgressive hybridization might have also occurred in relatively recent contact and potentially ongoing evolutionary event. The present study presents an example of historical hybridization and introgression at the southernmost distribution limits of Dolly Varden.     

Interspecific hybridization in Daphnia: distinction and origin of hybrid matrilines

Three coexisting Daphnia species belonging to the D. longispina group (D. galeata, D. hyalina, and D. cucullata) form species-hybrid complexes by producing interspecific hybrids in several lakes in Germany and The Netherlands. To evaluate the genetic consequences of interspecific hybridization, I studied the patterns of mitochondrial DNA (mtDNA) sequence variation. The directionality of interspecific hybridization and divergence of hybrids from parental species was tested, using the DNA sequences of a segment of mtDNA. Via the polymerase chain reaction, it was possible to investigate single animals and even single resting eggs. A species-specific marker was established, using restriction patterns of amplified cytochrome b segments. mtDNA genotypes of hybrids revealed unidirectional mitochondrial gene flow for two hybrids, which were investigated by using multiple clones. No evidence for introgression of mtDNA was found. On the basis of a phylogenetic analysis, the species exhibit considerable distinctness, whereas differences between clones within species and between hybrids and maternal species tend to be very low. These results indicate a recent origin of hybrids and suggest that the radiation of the D. longispina group occurred > 5 Mya.      

Evidence for historical introgression along a contact zone between two species of char (Pisces: Salmonidae) in northwestern North America

Phylogeographic analyses can yield valuable insights into the geographic and historical contexts of contact and hybridization between taxa. Two species of char (Salmonidae), Dolly Varden (Salvelinus malma) and bull trout (S. confluentus) have largely parapatric distributions in watersheds of northwestern North America. They are, however, sympatric in several localities and hybridization and some introgression occurs across a broad area of contact. We conducted a comparative phylogenetic analysis of Dolly Varden and bull trout to gain a historical perspective of hybridization between these species and to test for footprints of historical introgression. We resolved two major Dolly Varden mitochondrial DNA (mtDNA) clades (with 1.4-2.2% sequence divergence between haplotypes) that had different geographical distributions. Clade N is distributed across most of the range of Dolly Varden, from southern British Columbia through to the Kuril Islands in Asia. Clade S had a much more limited distribution, from Washington state, at the southern limit of the Dolly Varden range, to the middle of Vancouver Island. The distribution and inferred ages of the mtDNA clades suggested that Dolly Varden survived the Wisconsinan glaciation in a previously unsuspected refuge south of the ice sheet, and that Dolly Varden and bull trout were probably in continuous contact over most of the last 100,000 years. When bull trout were included in the phylogenetic analysis, however, the mtDNA of neither species was monophyletic: Clade S Dolly Varden clustered within the bull trout mtDNA clade. This pattern was discordant with two nuclear phylogenies produced (growth hormone 2 and rRNA internal transcribed sequence 1), in which Dolly Varden and bull trout were reciprocally monophyletic. This discordance between mtDNA- and nDNA-based phylogenies indicates that historical introgression of bull trout mtDNA into Dolly Varden occurred. Percent sequence divergence within these introgressed Dolly Varden (clade S) was 0.2-0.6%, implying that the introgression occurred prior to the most recent glaciation. Our analysis and other evidence of contact between divergent lineages in northwestern North America strongly suggests that the area may be the site of previously unsuspected suture zones of aquatic biotas.     

Large extent of mitochondrial DNA transfer from Oreochromis aureus to O. niloticus in West Africa

Introgressive hybridization has an important evolutionary significance in terms of gene diversity and speciation. Among the major groups of vertebrates, fish show a strong propensity to hybridize. In order to highlight the possible occurrence of gene flow between two tilapia species, Oreochromis niloticus and O. aureus, a comparison of allozyme and mitochondrial DNA (mtDNA) polymorphism was performed on sympatric and allopatric populations of these two species. Nuclear data were congruent with the morphological identification of O. niloticus and O. aureus populations. In opposition, the mtDNA analysis resulted in two strictly differentiated groups which did not follow the morphological and nuclear DNA classification. The first group consisted of East African O. niloticus populations and the second included all the O. aureus populations and the West African O. niloticus populations. Moreover, in some cases, the same sequences were detected in both species. These data strongly support a differential introgression of mtDNA from O. aureus to O. niloticus involving all the West African area. This work points out the risk of misinterpretation of mtDNA or nuclear DNA data when only one single class of marker is used.     

Recent introgressive hybridization revealed by exclusive mtDNA transfer from Oreochromis leucostictus (Trewavas, 1933) to Oreochromis niloticus (Linnaeus, 1758) in Lake Baringo, Kenya

Nuclear DNA and mtDNA polymorphisms were surveyed in various species of East African Oreochromis . In Lake Baringo, where only Oreochromis niloticus baringoensis is present, alien mtDNA haplotypes were observed, apparently the result of introgressive hybridization with Oreochromis leucostictus . This introgression is not accompanied by any substantial or recorded transfer of nuclear genes into O. n. baringoensis .     

Mitochondrial genome recombination in the zone of contact between two hybridizing conifers

Variation in mitochondrial DNA was surveyed at four gene loci in and around the zone of contact between two naturally hybridizing conifers, black spruce (Picea mariana) and red spruce (P. rubens) in northeastern North America. Most of the mtDNA diversity of these species was found in populations next to or into the zone of contact, where some individuals bore rare mitotypes intermediate between the common mitotypes observed in the allopatric areas of each species. Sequence analysis and tests for mtDNA recombination point to this phenomenon, rather than to recurrent mutation, as the most tenable hypothesis for the origin of these rare mitotypes. From the 10 mitotypes observed, at least 4 would be the product of recombination between 4 of the 5 putative ancestral mitotypes. Tests for cytonuclear disequilibrium and geographical structure of the putative recombinant mitotypes suggest that mtDNA recombination is not frequent and relatively recent on the geological time scale. mtDNA recombination would have been promoted by transient heteroplasmy due to leakage of paternal mtDNA since the Holocene secondary contact between the two species.     

Evidence for bimodal hybrid zones between two species of char (Pisces: Salvelinus) in northwestern North America

Dolly Varden (Salvelinus malma, Pisces: Salmonidae) and bull trout (Salvelinus confluentus) have widely overlapping, but largely parapatric ranges in watersheds in northwestern North America from Washington State to northern British Columbia. Genetic analysis of natural populations using diagnostic molecular markers revealed widespread local sympatry and hybridization with hybrids comprising 0-25% of the local samples. In a detailed analysis of hybridization using four nuclear DNA markers and mitochondrial DNA within the Thutade Lake watershed, northcentral British Columbia, hybrid genotypes constituted up to 9% of the population of juvenile char. There were significant deviations from Hardy-Weinberg, gametic, and cytonuclear equilibria, and local samples showed bimodal frequency distributions of genotypes. Pure parental and inferred backcross genotypes were most common, and F1 and F(n) hybrids were comparatively rare. Interspecific hybridization was asymmetrical, with most F1 hybrids (five of six) bearing S. confluentus mtDNA. The introgression of nuclear and mitochondrial alleles was asymmetrical, with S. confluentus mtDNA and Growth Hormone 2 introgressing into S. malma significantly more than either introgression of the three other nuclear loci, or introgression of S. malma alleles into S. confluentus. Substantial prezygotic isolation between the species likely depends on the large body size difference between them in sympatry: S. malma have small bodies and a stream resident life history (12-21 cm adult fork length at maturity), while S. confluentus are larger and adfluvial, i.e., they migrate to Thutade Lake where they grow to maturity before returning to tributary streams to spawn (40-90 cm at maturity). These traits may limit interspecific pairings because of size assortative pairing and size-dependent reproductive habitat use.     

Hybridization dynamics between sympatric species of trout: loss of reproductive isolation

Although reinforcement should enhance reproductive barriers in sympatric species, sympatric trout species do hybridize. Using mitochondrial and nuclear species markers, we investigated hybridization directionality, hybrid mating biases, and selection against hybrids in 13 sympatric cut-throat and rainbow trout populations on Vancouver Island, Canada. Approximately 50% of the genotyped fish were hybrid (F1 or higher-order) and populations ranged from very recent (all F1 hybrids) to extremely advanced higher-order hybridization. Overall, interbreeding was reciprocal, although some populations showed directional hybridization. Pronounced cytonuclear disequilibrium in post-F1 hybrids indicated a remarkable mating bias not previously reported, which is most likely because of behavioural reproductive preferences. Selection against hybrids was observed in only two populations, indicative of extrinsic selection. Two populations were 'hybrid swarms', with a complete loss of reproductive isolation. The complex hybridization dynamics in this system represent a valuable natural experiment of the genetic and evolutionary implications of recent and on-going interspecific hybridization.     

New evidence from mitochondrial DNA of a progenitor-derivative species relationship between black spruce and red spruce (Pinaceae)

Mitochondrial DNA (mtDNA) markers were used to assess the genetic diversity in allopatric populations of black spruce (Picea mariana [Mill.] BSP) and red spruce (P. rubens Sarg.). Patterns of mitochondrial haplotypes (mitotypes) were strikingly different between the two species. All mtDNA markers surveyed were polymorphic in black spruce, revealing four different mitotypes and high levels of mtDNA diversity (P(p) = 100%, A = 2.0, H = 0.496). In contrast, populations of red spruce had only two mitotypes and harbored low levels of ggenetic diversity (P(p) = 13.2%, A = 1.1, H = 0.120). When the southernmost allopatric populations of red spruce were considered, only one mitotype was detected. As previously reported for nuclear gene loci, the diversity observed for mtDNA in red spruce was a subset of that found in black spruce. Comparison of present and previously published data supports the hypothesis of a recent progenitor-derivative relationship between these species, red spruce presumably being derived by allopatric speciation of an isolated population of black spruce during the Pleistocene.     

Rare and asymmetrical hybridization of the endemic Barbus carpathicus with its widespread congener Barbus barbus

In endemic species that co-occur with widespread congeners, hybridization can lead to an influx of novel and beneficial genetic variation, but high rates of introgression may cause genetic swamping of the endemic species and have detrimental effects on its survival potential. This study examines hybridization between sympatric populations of the Carpathian barbel Barbus carpathicus , a recently discovered cryptic species with a restricted range, and the widespread common barbel Barbus barbus . Based on six diagnostic allozyme loci, a microsatellite locus and mtDNA, hybrids were found to be present at multiple localities within the Vistula River drainage (Baltic Sea) as well as in the Tisza River system of the Danube River drainage (Black Sea). However, the numbers of hybrids were very low; four individuals of 230 fish sampled from the Vistula drainage. Bayesian assessment of their nuclear genotypes suggested that two hybrids in the Vistula drainage and nine in the Tisza system were F₁ generation, and one in the Vistula drainage and one in the Tisza system were backcrosses (BC) to B. barbus , while no F₂ or BC to B. carpathicus were detected. No hybrid carried B. carpathicus mtDNA and cytonuclear linkage disequilibria showed significant positive associations between hybrid genotypes and B. barbus mtDNA, suggesting unidirectionality in the interspecific mating with a disproportionate contribution of B. barbus mothers. Despite geographically broad occurrence of hybrids, these data provide evidence of strong constraints on hybridization in the native breeding habitats and the lack of introgression towards B. carpathicus .     

Introgression and selection shaped the evolutionary history of sympatric sister‐species of coral reef fishes (genus: Haemulon)

Closely related marine species with large overlapping ranges provide opportunities to study mechanisms of speciation, particularly when there is evidence of gene flow between such lineages. Here, we focus on a case of hybridization between the sympatric sister‐species Haemulon maculicauda and H. flaviguttatum, using Sanger sequencing of mitochondrial and nuclear loci, as well as 2422 single nucleotide polymorphisms (SNPs) obtained via restriction site‐associated DNA sequencing (RADSeq). Mitochondrial markers revealed a shared haplotype for COI and low divergence for CytB and CR between the sister‐species. On the other hand, complete lineage sorting was observed at the nuclear loci and most of the SNPs. Under neutral expectations, the smaller effective population size of mtDNA should lead to fixation of mutations faster than nDNA. Thus, these results suggest that hybridization in the recent past (0.174–0.263 Ma) led to introgression of the mtDNA, with little effect on the nuclear genome. Analyses of the SNP data revealed 28 loci potentially under divergent selection between the two species. The combination of mtDNA introgression and limited nuclear DNA introgression provides a mechanism for the evolution of independent lineages despite recurrent hybridization events. This study adds to the growing body of research that exemplifies how genetic divergence can be maintained in the presence of gene flow between closely related species.     

Multiregional introgressions inferred from the mitochondrial DNA phylogeny of a hybridizing species complex of gobiid fishes, genus Tridentiger

Partial sequences of the cytochrome b gene (402 bp) in mtDNA were determined for brackishwater gobiid fishes, genus Tridentiger, collected from geographically distant locations in the Japanese Archipelago, and their interspecific and geographic variations were analyzed and compared. Contrary to the results of a previous allozyme analysis which revealed the existence of considerable genetic divergence (Nei's genetic distance > 0.5) between T. obscurus and T. brevispinis, the mtDNA haplotypes (mitotypes) of these two species were very similar and could not be distinguished by any of the neighbor- joining, maximum-likelihood or parsimony analyses. Hybrid individuals between the two species were also found, with several mitotypes being shared by both species and their hybrids. The phylogenetic relationships of mitotypes were divided into three subgroups, the geographical distributions of the latter being allied to geographical features of the Archipelago. These results suggested the occurrence of multiregional introgression between the two species, with mitotypes transferring from one species to the other.