Evan W. Carson

Historical Biogeography of the New-World Pupfish Genus Cyprinodon (Teleostei: Cyprinodontidae)

Abstract Analysis of mtDNA sequence variation (2,548 bp from ND2, cytb, and part of the control region) indicates that the genus Cyprinodon began diverging in the Late Miocene from a common ancestor with Megupsilon, a monotypic genus on the Mesa del Norte of Mexico. The geographic pattern of mtDNA variation, with estimates of divergence time, suggests that by the end of the Miocene Cyprinodon occurred from the Atlantic Coast and West Indies to near the western margin of North America via ancestral Rio Grande and Colorado River systems. Phylogeographic structure within the major mtDNA complexes supports a variety of hypotheses from geology and previous phylogenetic analyses for Late Neogene connections among basins in southwestern North America now separated by formidable barriers to dispersal. Comparison of the mtDNA tree with previous phylogenetic inferences from allozymes indicates that reticulate evolution involving divergent lineages probably was important in the history of Cyprinodon.

Influence of hydrogeographic history and hybridization on the distribution of genetic variation in the pupfishes Cyprinodon atrorus and C. bifasciatus

The evolutionary importance of hybridization in animals has been subject of much debate. In this study, we examined the influence of hydrogeographic history and hybridization on the present distribution of nuclear and mitochondrial DNA variation in two pupfish species, Cyprinodon atrorus and Cyprinodon bifasciatus. Results presented here indicate that there has been limited introgression of nuclear genes; however, mtDNA introgression has been substantial, with complete replacement of the C. bifasciatus mitochondrial genome by that of C. atrorus. Subsequent to this replacement, there has been diversification of mitochondrial haplotypes along major geographic regions in the basin. Evidence was also found that mitochondrial replacement follows a predictable, cyclical pattern in this system, with isolation and diversification followed by re‐contact and replacement of C. bifasciatus mitochondrial haplotypes by those of C. atrorus. This pattern is best explained by a combination of a numeric bias towards C. atrorus and mating site selection rather than selection for C. atrorus mitochondrial genome. These results demonstrate the important role hybridization can play in evolution.

Environmental variation, hybridization, and phenotypic diversification in Cuatro Ciénegas pupfishes

Hybridization can generate novel phenotypes, and in combination with divergent selection along environmental gradients, can play a driving role in phenotypic diversification. This study examined the influence of introgressive hybridization and environmental variation on the phenotypic diversity of two pupfish species (Cyprinodon atrorus and Cyprinodon bifasciatus) endemic to the Cuatro Ciénegas basin, Mexico. These species occupy opposite environmental extremes and are comprised of multiple, intraspecifically isolated populations. However, interspecific hybridization occurs to various degrees within connecting, intermediate environments. Using geometric morphometric analysis, extensive variation of body shape was observed between and within species, and phenotypic variation was strongly correlated with environmental conditions. Furthermore, some introgressed populations exhibited unique phenotypes not found in either of the parents, and overall morphospace occupation was significantly higher in introgressed populations when compared to the parentals. Overall, we find environmental variation and transgressive segregation both appear to have been important in shaping phenotypic variation in this system.

Genetic monitoring and complex population dynamics: insights from a 12-year study of the Rio Grande silvery minnow

The endangered Rio Grande silvery minnow persists as a remnant population in a highly fragmented and regulated arid‐land river system. The species is subject to dramatic annual fluctuations in density. Since 2003, the wild population has been supplemented by hatchery‐reared fish. We report on a 12‐year (1999–2010) monitoring study of genetic diversity and effective population size (Ne) of wild and hatchery stocks. Our goals were to evaluate how genetic metrics responded to changes in wild fish density and whether they corresponded to the number and levels of diversity of hatchery‐reared repatriates. Genetic diversity and all measures of Ne in the wild population did not correlate with wild fish density until hatchery supplementation began in earnest. Estimates of variance and inbreeding effective size were not correlated. Our results suggest source–sink dynamics where captive stocks form a genetically diverse source and the wild population behaves as a sink. Nevertheless, overall genetic diversity of silvery minnow has been maintained over the last decade, and we attribute this to a well‐designed and executed propagation management plan. When multiple factors like environmental fluctuation and hatchery supplementation act simultaneously on a population, interpretation of genetic monitoring data may be equally complex and require considerable ecological data.

Relationships between spatio-temporal environmental and genetic variation reveal an important influence of exogenous selection in a pupfish hybrid zone.

The importance of exogenous selection in a natural hybrid zone between the pupfishes Cyprinodon atrorus and Cyprinodon bifasciatus was tested via spatio‐temporal analyses of environmental and genetic change over winter, spring and summer for three consecutive years. A critical influence of exogenous selection on hybrid zone regulation was demonstrated by a significant relationship between environmental (salinity and temperature) and genetic (three diagnostic nuDNA loci) variation over space and time (seasons) in the Rio Churince system, Cuatro Ciénegas, Mexico. At sites environmentally more similar to parental habitats, the genetic composition of hybrids was stable and similar to the resident parental species, whereas complex admixtures of parental and hybrid genotypic classes characterized intermediate environments, as did the greatest change in allelic and genotypic frequencies across seasons. Within hybrids across the entire Rio Churince system, seasonal changes in allelic and genotypic frequencies were consistent with results from previous reciprocal transplant experiments, which showed C. bifasciatus to suffer high mortality (75%) when exposed to the habitat of C. atrorus in winter (extreme temperature lows and variability) and summer (abrupt salinity change and extreme temperature highs and variability). Although unconfirmed, the distributional limits of C. atrorus and C. atrorus‐like hybrids appear to be governed by similar constraints (predation or competition). The argument favouring evolutionary significance of hybridization in animals is bolstered by the results of this study, which links the importance of exogenous selection in a contemporary hybrid zone between C. atrorus and C. bifasciatus to previous demonstration of the long‐term evolutionary significance of environmental variation and introgression on the phenotypic diversification Cuatro Ciénegas Cyprinodon.

Time-series analysis reveals genetic responses to intensive management of razorback sucker (Xyrauchen texanus).

Time‐series analysis is used widely in ecology to study complex phenomena and may have considerable potential to clarify relationships of genetic and demographic processes in natural and exploited populations. We explored the utility of this approach to evaluate population responses to management in razorback sucker, a long‐lived and fecund, but declining freshwater fish species. A core population in Lake Mohave (Arizona‐Nevada, USA) has experienced no natural recruitment for decades and is maintained by harvesting naturally produced larvae from the lake, rearing them in protective custody, and repatriating them at sizes less vulnerable to predation. Analyses of mtDNA and 15 microsatellites characterized for sequential larval cohorts collected over a 15‐year time series revealed no changes in geographic structuring but indicated significant increase in mtDNA diversity for the entire population over time. Likewise, ratios of annual effective breeders to annual census size (Nb/Na) increased significantly despite sevenfold reduction of Na. These results indicated that conservation actions diminished near‐term extinction risk due to genetic factors and should now focus on increasing numbers of fish in Lake Mohave to ameliorate longer‐term risks. More generally, time‐series analysis permitted robust testing of trends in genetic diversity, despite low precision of some metrics.

Genetic Studies of Hatchery-Supplemented Populations of Red Drum in Four Texas Bays

Abstract Genetic diversity, population structure, average long-term effective population size (Ne ), and average long-term genetic migration rate of red drum Sciaenops ocellatus in each of four Texas bays were assessed using variation in 13 nuclear-encoded microsatellites among samples from the 2004 and 2005 cohorts. No significant differences in genetic diversity were detected among bays. Levels of gene diversity of red drum in each bay were equal to or greater than estimates reported for microsatellites in red drum sampled previously from two of the four bays and from other bays in the southeastern USA, including some that had not yet been supplemented with hatchery-raised fish. Tests of the homogeneity of allele and genotype distributions (including analysis of molecular variance) among the four bays were nonsignificant. Estimates of the migration rate (m) between bays ranged from 0.08% to 0.15%, with the average long-term number of migrants (calculated as Ne × m) between bays estimated to range from 1...

Importance of Exogenous Selection in a Fish Hybrid Zone: Insights from Reciprocal Transplant Experiments

Abstract A key element in understanding hybridization in fishes is to determine the relative influences of endogenous and exogenous selection on hybrid zone structure. To date, such studies have been limited. To help fill this gap, we conducted reciprocal transplant experiments with two pupfish species, Cyprinodon atrorus and C. bifasciatus, to assess the importance of environmental factors in their hybridization dynamics. Results from our experiments show that C. atrorus survives well when environmental variability is low or high, whereas C. bifasciatus only survives well when environmental variability is low. This corroborates the hypothesis that C. atrorus is euryplastic and C. bifasciatus stenoplastic, and also provides evidence that differential environmental tolerance may be important in regulating the distribution of genetic variation in hybrid zones between these species. However, these differences do not explain why C. atrorus is absent from C. bifasciatus habitats. This absence is likely determined at least in part by other exogenous selection factors, such as disproportionate predator susceptibility and/or inferior competitive ability of C. atrorus. Although the relative role of endogenous selection in this system remains unknown, this study provides insight into the potential complexity of exogenous selection in fish hybrid zones.

Conservation Genetics of Cyprinid Fishes (Genus Dionda) in Southwestern North America. II. Expansion of the Known Range of the Manantial Roundnose Minnow, Dionda argentosa

Abstract Cyprinids belonging to the genus Dionda have a relatively broad distribution in southwestern North America. However, validity of populations in several of the nominal species has not been examined rigorously. During the course of a conservation-genetics study of Dionda in western Texas we determined that one population presumed to represent Dionda episcopa instead belongs to Dionda argentosa. This extends the native range of D. argentosa to include the lower Pecos River basin in addition to its currently recognized distribution in three tributaries of the Rio Grande. Conservation implications for both species are discussed.

Threatened fishes of the world: Cyprinodon bifasciatus Miller 1968 (Cyprinodontidae)

Common Names: Cuatrociengas pupfish, cachoritto de Cuatrociengas. Conservation Status: Listed as Threatened (SEMARNAT 2002). Current IUCN Red List status: Lower Risk/least concern. Identification: Morphologically streamlined. Mature males are chalky blue, have yellow eyes, and a thin, terminal black band on the caudal fin. Females and juveniles have two dark brown lateral lines, with a light blue to pale brown base color. Juveniles and adults have no ocellus on dorsal or anal fin. Mature males range from 30.9 to 51.4 mm SL, mature females from 29.5 to 41.3 mm SL (Miller 1968). Image: modification of mature male holotype in Miller (1968). Distribution: Endemic to the Cuatrociengas basin, Coahuila, Mexico. Abundance: Abundant, with generally large and stable populations throughout its range. Habitat and ecology: Inhabits stable, physicochemically benign thermal springs, outflows, and rivers (Miller 1968; Minckley 1969). Young are omnivorous, but diet is largely plant/ sediment based after 24–29 mm SL (Arnold 1972). Co-occurs with many fish species (Miller 1968) and is stenoplastic (Carson et al. 2008). Often sympatric with C. atrorus in intermediate environments, where natural hybridization is extensive. Ancient hybridization led to complete replacement of C. bifasciatus mitochondrial genome by that of C. atrorus, but nuclear gene introgression from C. atrorus appears minor to absent (Carson and Dowling 2006). Reproduction: Mature males breed and feed in leks in shallow water. Mature females school and feed in deeper water, entering leks to spawn. Breeding occurs year-round, though is reduced in winter (Arnold 1972). Threats: Groundwater extraction is primary. Expanded eco-tourism and introduction of non-native species are also of concern. Conservation action: In 1994 the Cuatrociengas basin was declared a Natural Protected Area by the Mexican government. Conservation efforts since have been weakly-effective (Figueroa and Sanchez-Cordero 2008). Conservation recommendations: Elimination of groundwater extraction is urgently needed. Effective conservation strategies need to be devised and implemented. Environ Biol Fish (2009) 86:445–446 DOI 10.1007/s10641-009-9546-8