Bruce J. Turner

A mixed-mating strategy in a hermaphroditic vertebrate

Mixed-mating systems, in which hermaphrodites can either self-fertilize or outcross, are common in many species of plants and invertebrates and have been informative models for studying the selective forces that can maintain both inbreeding and outbreeding in populations. Here, we document a remarkable instance of evolutionary convergence to an analogous mixed mating system by a vertebrate, the mangrove killifish (Kryptolebias marmoratus). In this androdioecious species, most individuals are simultaneous hermaphrodites that characteristically self-fertilize, resulting in local populations that consist of (nearly) homozygous lines. Most demes are also genetically diverse, an observation traditionally attributed to de novo mutation coupled with high levels of inter-site migration. However, data presented here, from a survey of 35 microsatellite loci in Floridian populations, show that genotypic diversity also stems proximally from occasional outcross events that release ‘explosions’ of transient recombinant variation. The result is a local population genetic pattern (of extensive genotypic variety despite low but highly heterogeneous intra-individual heterozygosities) that differs qualitatively from the genetic architectures known in any other vertebrate species. Advantages of a mixed-mating strategy in K. marmoratus probably relate to this fishs solitary lifestyle and its ability to colonize new habitats.

Evolutionary genetics of Death Valley pupfish populations: mitochondrial DNA sequence variation and population structure

The pupfishes of Death Valley (genus: Cyprinodon) are a classic example of rapid (postPleistocene) allopatric divergence with several populations exhibiting striking morphological, physiological, and behavioural differences. However, genetic changes accompanying or causing this divergence have been poorly resolved and/or difficult to detect. In order to investigate the phylogeography of the system and infer historical effective population sizes, mtDNA sequence variation was assessed within and among populations. In total, mtDNA D-loop sequences revealed 11 haplotypes among 278 individuals from 16 populations. Genetic diversity within populations was generally low (1–3 haplotypes per population) suggesting relatively small effective population sizes. Most variation occurred among populations, resulting in extensive genetic structure. The genealogical relationships of mtDNA haplotypes were determined by a combined phylogenetic analysis of both D-loop and ND2 sequences. These relationships revealed that the present distribution of haplotypes in Death Valley has probably resulted from stochastic, and in some cases, incomplete sorting of ancestral variation. The presence of highly divergent haplotypes in some populations may indicate that ancestral population sizes were substantially larger, perhaps rivalling population sizes observed in contemporary coastal pupfish species. In contrast, other Death Valley populations of equal contemporary size appeared monomorphic suggesting historical genetic bottleneck events. The Death Valley pupfishes provide an important example of the limitations inherent in defining evolutionarily significant units (ESUs) for conservation purposes based on mtDNA sequence variation alone.

A Novel Terrestrial Fish Habitat inside Emergent Logs

Reports of new habitats for a major group of organisms are rare. Fishes display diverse adaptations for temporary (amphibious) existence on land, but to our knowledge, none have ever been reported regularly living inside emergent logs. Here, we show that the mangrove killifish, Kryptolebias marmoratus, a species previously known to emerse (leave the water) regularly, is now known to emerse and aggregate in large numbers inside decaying mangrove logs that have been “galleried” by terrestrial insects. This behavior has now been documented in both Belize, Central America, and Florida, U.S.A., populations and represents the first known case of fishes entering terrestrial woody material. The dense packing of fish in the narrow log galleries may imply a novel social context in which intraspecific aggressive behaviors are reduced, possibly mediated by the physiological limitations imposed within this restrictive habitat.

Temperature preferences and tolerances of three fish species inhabiting hyperthermal ponds on mangrove islands

The fish species Cyprinidon artifrons, Floridichthys carpio, and Gambusia yucatana inhabit shallow mangrove ponds off the coast of Belize. Portions of these ponds experience a diurnal temperature change from 26 °C at night to 40 °C and above during midday. Repeated field observations indicate Cyprinidon prefer the warmer (and much larger) portions of the ponds whereas the other two species stay in the cooler areas.The hypothesis that temperature is serving as a cue for partitioning within the ponds was supported by laboratory thermal gradient tests in which Cyprinidon preferred temperatures clearly higher than the other two species.The critical thermal maximum (CTM) was determined for the three species using members that had been acclimated to either a daily cycling temperature similar to that for the ponds, or to the mean of the 24-hour cycle (30 °C). Cyprinidon acclimated to the cycling temperature had a CTM of 45.5 °C, which apparently sets a new record for fish CTM. Acclimation to a constant 30 °C lowered the CTM to 43.7 °C. Floridichthys and Gambusia acclimated to the cycled temperature had CTMs of 43.9 and 43.3 °C respectively, and 42.5 and 42.6 °C for those acclimated to 30 °C.All three species appear to have the ability to tolerate the high temperatures throughout the ponds but only Cyprinidon utilize the whole pond during the day. This may help to explain the large populations of Cyprinodon found in these mangrove ponds compared to the other species.

Use of Genetic Tags to Evaluate Stocking Success for Reservoir Walleyes

Abstract In recent years, recruitment of walleyes Stizostedion vitreum vitreum in Claytor Lake, Virginia, has resulted from both natural reproduction and periodic stockings of age-0 fish from the midwestern United States. Malate dehydrogenase (MDH) allele frequencies of six walleye cohorts were assessed by starch gel electrophoresis. Augmented cohorts were found to possess MDH allele frequencies significantly different from unstocked cohorts, and both augmented and unstocked cohorts differed in allele frequencies from the juvenile fish that had been stocked during that period. Success of supplemental stocking was evaluated by quantifying shifts in cohort allele frequencies due to the stocking of juvenile fish with allele frequencies different from resident fish. Supplemental stocking of juvenile walleyes contributed an average 67% of year-class strength to augmented cohorts in Claytor Lake during 1976–1979. This investigation demonstrates the utility of genetic tags for evaluating stocking efficiency, eve...

Multiple paternity in the red‐eyed treefrog Agalychnis callidryas (Cope)

DNA fingerprinting of tadpoles from two different ‘one female‐two male’ matings of the red‐eyed treefrog, Agalychnis callidryas, revealed multiple paternity of offspring. Offspring were assigned paternity based on bands shared with the putative father, but not shared between putative fathers or with the mother. Paternity was split 44/56% and 36/64% in two matings. These results do not support a hypothesis of sperm priority in access to unfertilized eggs by primary males. Multiple paternity may be commonplace in species of anurans with matings by multiple males.

Strong population structure despite evidence of recent migration in a selfing hermaphroditic vertebrate, the mangrove killifish ( Kryptolebias marmoratus )

We employ a battery of 33 polymorphic microsatellite loci to describe geographical population structure of the mangrove killifish (Kryptolebias marmoratus), the only vertebrate species known to have a mixed‐mating system of selfing and outcrossing. Significant population genetic structure was detected at spatial scales ranging from tens to hundreds of kilometres in Florida, Belize, and the Bahamas. The wealth of genotypic information, coupled with the highly inbred nature of most killifish lineages due to predominant selfing, also permitted treatments of individual fish as units of analysis. Genetic clustering algorithms, neighbour‐joining trees, factorial correspondence, and related methods all earmarked particular killifish specimens as products of recent outcross events that could often be provisionally linked to specific migration events. Although mutation is the ultimate source of genetic diversity in K. marmoratus, our data indicate that interlocality dispersal and outcross‐mediated genetic recombination (and probably genetic drift also) play key proximate roles in the local ‘clonal’ dynamics of this species.

Diabolical survival in Death Valley: recent pupfish colonization, gene flow and genetic assimilation in the smallest species range on earth.

One of the most endangered vertebrates, the Devils Hole pupfish Cyprinodon diabolis, survives in a nearly impossible environment: a narrow subterranean fissure in the hottest desert on earth, Death Valley. This species became a conservation icon after a landmark 1976 US Supreme Court case affirming federal groundwater rights to its unique habitat. However, one outstanding question about this species remains unresolved: how long has diabolis persisted in this hellish environment? We used next-generation sequencing of over 13 000 loci to infer the demographic history of pupfishes in Death Valley. Instead of relicts isolated 2–3 Myr ago throughout repeated flooding of the entire region by inland seas as currently believed, we present evidence for frequent gene flow among Death Valley pupfish species and divergence after the most recent flooding 13 kyr ago. We estimate that Devils Hole was colonized by pupfish between 105 and 830 years ago, followed by genetic assimilation of pelvic fin loss and recent gene flow into neighbouring spring systems. Our results provide a new perspective on an iconic endangered species using the latest population genomic methods and support an emerging consensus that timescales for speciation are overestimated in many groups of rapidly evolving species.

Homozygosity and heterozygosity in three populations of Rivulus marmoratus

Rivulus marmoratus is a self-fertilizing hermaphroditic fish found in the tropical Atlantic as populations of homozygous clones, with the exception of a single site in Belize where male fish are abundant and heterozygosity is the norm. The presence of male fish apparently leads to outcrossing and heterozygosity, but males have been found in limited numbers in other populations which are homozygous. DNA fingerprinting now reveals that the Belize population has remained heterozygous, with a high proportion of males (20–25%), for several years. In addition, two newly discovered populations with a lower percentage of males (1–2%) are reported from the Bahamas and Honduras. One of these populations (Bahamas) consists of homozygous clones, while the other (Honduras) displays a limited proportion of heterozygosity. The Honduras population is only the second outcrossing population known in this species, and the limited heterozygosity seen here may reflect the lower proportion of males.

EVIDENCE THAT AN OUTCROSSING POPULATION IS A DERIVED LINEAGE IN A HERMAPHRODITIC FISH (RIVULUS MARMORATUS)

Rivulus marmoratus is the only known vertebrate with obligate, synchronous hermaphroditic fertilization. Males can be experimentally induced in the laboratory and are rare or absent in most populations, but at the isolated Twin Cays, Belize, locality, males are relatively abundant. At this locality, evidence of outcrossing has been documented in this otherwise automictic cloning species. Phylogenetic analysis of restriction sites and sequence characters revealed that all Florida and Belize western Caribbean populations (including Twin Cays) are phyletically indistinguishable yet divergent from eastern populations in Brazil and the Bahamas. Further, these western lineages shared a common ancestor more recently than all other populations. Therefore, the Twin Cays population is not a remnant ancestral outcrossing population. Outcrossing is suspected to have evolved as a phenotypically plastic character, and its expression in R. marmoratus may be dormant unless triggered by some ecological factor that is not well understood.