Molecular systematics of the North American chub genus Macrhybopsis (Teleostei: Cyprinidae).

The North American fish genus Macrhybopsis (Teleostei: Cyprinidae) as presently conceived comprises 12 species and occurs in much of interior eastern North America. Variation in the mitochondrial ND2 gene and the nuclear S7 intron 1 reveal conflicting gene-tree relationships for deeper nodes, which are assumed to represent past introgression and heterospecific mitochondrial fixation. The results support monophyly for the wide-ranging M. aestivalis complex with successive sister relationships to M. gelida, M. meeki, and M. storeriana. The current species-level taxonomy of Macrhybopsis is generally supported. Species status is supported for the morphologically distinct M. australis and M. tetranema, both of which are genetically introgressed by M. hyostoma. The results agree with previous suggestions that the wide-ranging M. hyostoma harbors cryptic species. Similar crypticity is indicated for the poorly sampled M. storeriana; a sample from the Pearl River shows 8% ND2 divergence from two Mississippi River populations. Within the M. aestivalis complex, there are only two examples of geographic overlap among mtDNA phylogroups. One involves co-occurrence of the highly divergent M. marconis and M. cf. hyostoma, and the other is the detection of the apparently anthropogenic occurrence of mitochondrial DNA from a Red River form, either M. cf. hyostoma or M. australis, in the Cimarron River of the Arkansas River basin.

Molecular systematics and historical biogeography of the Nocomis biguttatus species group (Teleostei: Cyprinidae): nuclear and mitochondrial introgression and a cryptic Ozark species.

The Nocomis biguttatus species group ranges widely across North America from the Red River in Oklahoma and Arkansas north to Minnesota and east-west from Wyoming to Ontario. The group includes three traditionally recognized allopatric species: the wide-ranging N. biguttatus and two geographically more restricted species, N. asper from the western Ozarks (Arkansas River system) and two disjunct locations in the Red River system, and N. effusus from the Green, Cumberland, and lower Tennessee rivers. Separate analyses of the mitochondrial cytb gene and two nuclear genes (S7 intron 1 and a portion of the gene for growth hormone, GH), each resolved a cryptic species previously treated as N. biguttatus from the southern Ozarks (White River). Relationships among the four species were unresolved because of conflicts between cytb and S7 and a lack of resolution for GH. A previously indicated N. biguttatus-N. effusus sister-relationship appears to reflect past hybridization and mtDNA capture by N. effusus. Nocomis biguttatus includes four primary cytb clades with unresolved inter-relationships. A Northern Ozarks-Great Plains-Upper Midwest Clade and an Ohio River-Eastern Great Lakes Clade presumably represent late Quaternary dispersal from glacial refugia in, respectively, the northern Ozarks and an unglaciated portion of the Ohio River system. Other clades include one from the Meramec River and a Black River-St. Francis River Clade. There was evidence in N. effusus for a phylogeographic break between the lower Tennessee River and the Green-Cumberland basins. Geographic structure is weak in N. asper, indicating relatively recent contact between now disjunct populations in the Arkansas and Red river basins. The Blue River population of N. asper appears to reflect late Pleistocene or Holocene hybridization and genetic swamping of a resident native population of N. biguttatus by an invading population of N. asper. This postulates past occurrence of N. biguttatus far south of its present range.

Molecular systematics of Hispaniolan pupfishes (Cyprinodontidae: Cyprinodon): implications for the biogeography of insular Caribbean fishes.

We used sequence variation in the mtDNA control-region and ND2 and cyt b genes to assess the systematics and biogeography of the five species of pupfish (Cyprinodon) on Hispaniola. These include four endemics, the relatively large-bodied Cyprinodon bondi, Cyprinodon nichollsi, and Cyprinodon sp., each from a separate lake in southwestern Hispaniola, and Cyprinodon higuey from a coastal lake in eastern Hispaniola. The fifth species consists of coastal populations referable to Cyprinodon variegatus riverendi. The results indicate that Hispaniola has been invaded by at least two forms, first by a late Pliocene progenitor of Cyprinodon variegatus ovinus and the large-bodied Hispaniolan species, and, more recently, by one or more ancestral forms allied with Cyprinodon variegatus variegatus and C. v. riverendi. Levels of divergence indicate that large expanses of open sea have not acted as long-term barriers to inter-island dispersal of cyprinodontiform fishes. This study, together with the molecular systematics of other insular Caribbean fishes, indicates that most insular groups originated from late Neogene dispersal from the mainland. The patterns of mtDNA variation in Cyprinodon showed little congruence with the species/subspecies taxonomy.

DEVELOPMENT OF THE HYBRID SWARM BETWEEN PECOS PUPFISH (CYPRINODONTIDAE: CYPRINODON PECOSENSIS) AND SHEEPSHEAD MINNOW (CYPRINODON VARIEGATUS): A PERSPECTIVE FROM ALLOZYMES AND mtDNA.

A comparison of allozyme and mtDNA frequencies was used for insight into a situation in the Pecos River, Texas where contact between the endemic pupfish (Cyprinodon pecosensis) and an introduced congener (C. variegatus) has resulted in rapid, geographically extensive genetic introgression. Temporal changes in mean frequencies of diagnostic allozyme markers indicate that the clinal pattern of introduced genetic material (Echelle and Connor 1989) is slowly decreasing in amplitude. Significant rank concordance in diagnostic allele frequencies among sites and across sampling years indicates directional influences upon temporal allele frequency change. These observations are consistent with the theory of gene flow in neutral clines. Levels of introgression indicated by each of four allozyme loci and mtDNA were roughly equivalent. The early history of the hybrid swarm is explained by genetic swamping, possibly mediated by selection for C. variegatus or C. variegatus × C. pecosensis, at a time when the normally abundant endemic species had been catastrophically depleted. High frequencies of an introduced GPI-A allele in all samples of intergrades suggests that the introduced genome originated with a single founding event.

MITOCHONDRIAL DNA VARIATION AND EVOLUTION OF THE DEATH VALLEY PUPFISHES (CYPRINODON, CYPRINODONTIDAE).

A phylogenetic analysis of mitochondrial DNA (mtDNA) restriction sites was used to examine the geographic history of the Cyprinodon nevadensis complex of pupfishes, a group of four species (seven extant subspp.) in two endorheic (closed) basins of the Death Valley System in California and Nevada (Owens River Valley and Ash Meadows-Death Valley). The mtDNA results suggest that the group contains mtDNAs from two divergent clades. One such clade is represented by the mtDNAs of the Owens Valley pupfish (C. radiosus) and the existing species in the Colorado River (C. macularius), while the other includes the mtDNAs of the Ash Meadows-Death Valley species (C. nevadensis, C. salinus, and C. diabolis) and a species located much farther to the east (C. fontinalis from the Guzman Basin, Chihuahua, Mexico). These results, together with evidence from other studies, suggest two separate invasions of the Death Valley System by pupfishes carrying phylogenetically divergent mtDNAs. The C. nevadensis complex apparently is either an artificial group or else it is monophyletic and its genetic history includes loss of the original mtDNA in either Owens Valley or Ash Meadows-Death Valley following genetic introgression after an invasion by a pupfish carrying a divergent mtDNA.

MITOCHONDRIAL-DNA DIVERSITY AND THE ORIGIN OF THE MENIDIA CLARKHUBBSI COMPLEX OF UNISEXUAL FISHES (ATHERINIDAE).

Restriction-endonuclease analysis of mitochondrial DNA (mtDNA) of the unisexual M. clarkhubbsi complex and close sexual relatives indicated that the unisexual complex arose through multiple, nonreciprocal hybridizations involving females of M. peninsulae. High-resolution analyses using restriction endonucleases that cleave at 4-bp sites revealed mtDNA sequence diversity that was low among unisexuals but high among individuals of M. peninsulae. The identification of M. peninsulae as a parent of the unisexuals conflicts with some details of previous allozyme comparisons. One possibility is that the unisexuals were derived from hybridization involving M. beryllina and a recently extinct form of M. peninsulae. In contrast to the unisexuals, contemporary hybrids of M. peninsulae and M. beryllina are formed by reciprocal matings. The origins of extant unisexual lineages from nonreciprocal hybridizations, together with their low mtDNA diversity relative to the maternal ancestor, implies strong constraints on origins of unisexuality via hybridization. Data on reproduction by contemporary F1 hybrids reveal one form of genetic/developmental constraint: M. peninsulae and M. beryllina may now have diverged beyond the point where the hybrid origin of new unisexual lineages is possible.

RAPID, GEOGRAPHICALLY EXTENSIVE GENETIC INTROGRESSION AFTER SECONDARY CONTACT BETWEEN TWO PUPFISH SPECIES (CYPRINODON, CYPRINODONTIDAE).

Apparently between 1980 and 1984, Cyprinodon variegatus was introduced into an area of the Pecos River in Texas, where it hybridized with an endemic species, C. pecosensis. Protein electrophoresis indicated that, by 1985, panmictic admixtures of these two pupfishes occupied approximately 430 river-kilometers of the Pecos River, roughly one-half of the historic range of the endemic species. The average frequency of introduced alleles at four diagnostic loci ranged from 0.18 to 0.84 at the 15 sites sampled from the Pecos River in Texas. Clinal patterns in allele frequencies suggest that C. variegatus was introduced into a mid-reach of the river and that this was followed by both upstream and downstream dispersal of the introduced alleles. All pairwise combinations of loci showed significant linkage disequilibrium. The level of disequilibrium indicates chromosomal linkage for one gene-pair, Gpi-A and Est-1. The change in pupfish allele frequencies in the Pecos River represents an extreme example of rapid natural selection in a seminatural situation.