Robert T. Dillon

Hard clam, Mercenaria mercenaria, broodstocks: Genetic drift and loss of rare alleles without reduction in heterozygosity☆

Abstract Two nursery stocks of the hard clam, Mercenaria mercenaria, selected for fast growth were compared to corresponding wild populations in regard to allele frequencies at seven polymorphic enzyme loci. Although as few as 30–60 parents were spawned at each of four generations to produce these two broodstocks, neither line exhibited any reduction in heterozygosity. Both lines, however, showed evidence of genetic drift and loss of rare alleles, suggesting that a cross between them could result in a third genetically distinct line.

Realized heritability of growth rate in the hard clam Mercenaria mercenaria

Abstract Native South Carolina wildstock clams ( Mercenaria mercenaria ) were mass-spawned to produce a large initial population of parents for a directed breeding program. At 2 years of age, the largest 10% of this population, and an equal number of mean size clams, were segregated to become selected and control-line parents. Three separate experiments were performed, usually involving 20–40 selected and control parents. Offspring were reared under standard hatchery, nursery and field grow-out conditions. Realized heritability was determined at 2 years of age. In one experiment, no response to selection was observed at 2 years, possibly due to reduced effective breeding number. Conservative estimates of realized heritability of growth rate for the other two experiments were consistent and high: 0.42±0.10 and 0.43±0.06. Mass selection appears to be a promising technique for improvement of hard clam broodstocks.

The snails the dinosaurs saw: are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?

Abstract Pleurocerid snails are a common element of the benthos in rivers and streams throughout the Appalachian highlands from Virginia to Georgia on both sides of the continental divide. Yet their dispersal capabilities are so limited today that significant gene frequency differences have been demonstrated over a scale of meters. We obtained mitochondrial cytochrome oxidase c subunit I (COI) sequence data from 3 individual snails sampled from each of 13 populations of pleurocerids representing 3 species—Leptoxis carinata (4 populations), Goniobasis (“Elimia”) catenaria (4 populations), and Goniobasis (“Elimia”) proxima (5 populations). To these data we added previously published COI sequences from 3 other G. proxima populations. Levels of intrapopulation sequence divergence were strikingly high, ranging up to 21.9% within populations and 22.6% between populations within species. A neighbor-joining analysis revealed 3 loose clusters corresponding to the species, but featured numerous extreme outliers. Wilcoxon rank-sum tests returned no evidence that the Continental Divide (as it presently stands) makes any contribution to mean levels of interpopulation sequence divergence, nor that simple geographic distance (regardless of modern drainage) has an effect. We suggest that populations of pleurocerid snails inhabiting the Older Appalachians might date to such an age that all geographic signal in the divergence of our test gene has been lost. We review additional lines of evidence from other genetic studies and from ecology, life history, continental biogeography, and the fossil record that suggest that our pleurocerid populations might be living fossils from the Paleozoic uplift of the Appalachians.

Hard clam, Mercenaria mercenaria, broodstocks: growth of selected hatchery stocks and their reciprocal crosses

Abstract Crosses between and within two hatchery stocks of hard clams, Mercenaria mercenaria , were created in three independent experiments and the offspring reared to 2 years of age. Environmental variation between experiments strongly influenced early growth, but mean sizes at 2 years were similar for the three experiments. Offspring from all crosses had above-average growth rates. Offspring were assessed electrophoretically at seven enzyme loci, and gene frequencies and overall heterozygosities were compared to parental stocks. There was no correlation between heterozygosity and size at 2 years, nor between heterozygosity and variance of mean size. Reciprocal crosses were not consistently faster growing nor more heterozygous than purebred lines, nor was any relationship apparent between cross and variance of mean size. However, offspring from the reciprocal crosses were genetically distinct from each other and from the purebred lines and possessed the desirable traits of the parental stocks.

Estimates of natural allosperm storage capacity and self- fertilization rate in the hermaphroditic freshwater pulmonate snail, Physa acuta

Summary Previous estimates of allosperm storage capacities and self-fertilization rates in pulmonate snails have been derived almost exclusively from laboratory mating experiments. Here we report observations on a single adult albino Physa acuta collected from a natural pond in Charleston, South Carolina. Cultured in isolation from August 2002 until its death in February, this individual laid an average of 78.3 eggs per week (s.e.m. = 6.2), with a hatching success of 35.1% and an overall frequency of albino (putatively selfed) offspring of 6.4%. We found no evidence of reduced fecundity, reduced offspring viability, or increased self-fertilization over 20 weeks of observation, suggesting that stores of allosperm at no time approached exhaustion. We suggest that self-fertilization in primarily outcrossing populations of pulmonate snails may not be an adaptation, but rather may be a consequence of inefficiency in the mechanism sorting autosperm from allosperm.

EMPIRICAL ESTIMATES OF REPRODUCTIVE ISOLATION AMONG THE FRESHWATER PULMONATE SNAILS PHYSA ACUTA, P. POMILIA, AND P. HENDERSONI

Physa hendersoni collected from its type locality near Yemassee, South Carolina, and Physa pomilia from its type locality near Claiborne, Alabama, both display the penial morphology that has been characterized as “type-bc” by Te (1978, 1980). Mate choice tests returned no evidence of premating reproductive isolation between these two populations, and no-choice breeding experiments confirmed outcross fecundity, F1 viability and F1 fertility comparable to incross controls. Significant premating reproductive isolation was documented, however, between the P. hendersoni population and a population of Physa acuta from Charleston, South Carolina, bearing the “type-c” penial morphology. No-choice breeding experiments involving P. acuta and P. hendersoni yielded a mixture of hybrid and selfed progeny, the hybrids apparently sterile. Thus the nomen Physa hendersoni is a junior synonym of P. pomilia, whereas P. pomilia and P. acuta are distinct biological species.

Genetics, Shell Morphology, and Life History of the Freshwater Pulmonate Limpets Ferrissia rivularis and Ferrissia fragilis

ABSTRACT The weak and variable nature of the shell morphometric criteria by which the North American ancylid limpets Ferrissia rivularis and Ferrissia fragilis are distinguished has led to speculation that they may be ecophenotypic variants. The shells of F. rivularis that we collected from a rocky river in the upper piedmont of South Carolina were significantly higher and wider per length than those of an F. fragilis population collected from a weedy ditch in the coastal plain. Both populations laid singleton eggs approximately 0.6 mm diameter in culture, hatching in one week and maturing in four-five weeks at 2.0–3.0 mm shell length. Adult F. rivularis reared in our laboratory environment were morphologically indistinguishable from F. fragilis, suggesting that the differences in shell shape displayed by their wild-collected parents may have little additive genetic component. Both populations were fixed for identical allozyme alleles at nine loci, with two alleles segregating at a tenth, IsdhF. The absence of ZsdhF heterozygotes in wild-collected samples of 104 F. rivularis individuals (p = 0.52, q = 0.48) and 102 F. fragilis individuals (p = 0.14, q = 0.86), together with no evidence of outcrossing in laboratory breeding experiments, suggests obligate self-fertilization. Thus in the absence of any reliable morphological distinction, we propose that F. fragilis be considered a junior synonym of F. rivularis.

The influence of minor human disturbance on biochemical variation in a population of freshwater snails

Abstract A 1979 survey of gene frequencies at the octopine dehydrogenase locus in a population of the snail Goniobasis proxima uncovered significant heterogeneity within a 500 m stretch of stream, apparently due to blockage of gene flow by a culvert. Minor earthwork and local siltation in 1980 resulted in homogenisation of gene frequencies, but the 1985 significant differences had been restored. Thus apparently minor environmental alterations seem to have influenced the evolution of a natural population.

NO REPRODUCTIVE ISOLATION BETWEEN FRESHWATER PULMONATE SNAILS PHYSA VIRGATA AND P. ACUTA

Abstract Mate choice tests provided no evidence of prezygotic reproductive isolation between a population of Physa virgata (Gould, 1855) collected from its type locality in the Gila River of Arizona and Physa acuta (Draparnaud, 1805) from a control site in Charleston, South Carolina. Reared in a no-choice experimental design, 10 outcross Arizona × South Carolina pairs initiated reproduction at approximately the same age as Arizona × Arizona controls, and earlier than South Carolina × South Carolina controls. Parents in the outcross experiment did not differ significantly from either control in their median weekly fecundity across 10 weeks of observation, yielding an F1 generation with significantly improved viability. We detected no evidence of reduction in F1 fertility. Thus, P. virgata, the most widespread freshwater gastropod of the American Southwest, should be considered a junior synonym of the cosmopolitan P. acuta.

Enzyme heterozygosity and growth rate in nursery populations of Mercenaria mercenaria (L.)

Abstract Crosses between and within two lines of the hard clam Mercenaria mercenaria (L.) were reared to age 1 yr under typical commercial upflow nursery conditions. Within these crosses, little relationship was detected between shell length and heterozygosity averaged over seven enzyme loci. However, significant differences between the largest and smallest clams were detected at individual loci in 10 of 42 tests. Results were consistent neither with the hypothesis that the alleles themselves were affecting growth, nor with the hypothesis that these enzyme loci are tightly linked to other loci affecting growth. Rather, it appears that alleles are marking the entire genomes of their parents, and that variation in the growth rates of the offspring from individual clams may be obscuring any relationship with overall heterozygosity.