Inheritance of juvenile shell colour of the oyster drill Urosalpinx cinerea

Shell color polymorphism in marine gastropods

Marine gastropods are characterized by an incredible variation in shell color. In this review, we aim to introduce researchers to previous studies of shell color polymorphism in this group of animals, trying to provide an overview of the topic and highlighting some potential avenues for future research. For this, we tackle the different aspects of shell color polymorphism in marine gastropods: its biochemical and genetic basis, its patterns of spatial and temporal distribution, as well as its potential evolutionary causes. In particular, we put special emphasis on the evolutionary studies that have been conducted so far to reveal the evolutionary mechanisms responsible for the maintenance of shell color polymorphism in this group of animals, as it constitutes the least addressed aspect in existing literature reviews. Several general conclusions can be drawn from our review: First, natural selection is commonly involved in the maintenance of gastropod color polymorphism; second, although the contribution of neutral forces (gene flow-genetic drift equilibrium) to shell color polymorphism maintenance do not seem to be particularly important, it has rarely been studied systematically; third, a relationship between shell color polymorphism and mode of larval development (related to dispersal capability) may exist. As for future studies, we suggest that a combination of both classical laboratory crossing experiments and -Omics approaches may yield interesting results on the molecular basis of color polymorphism. We believe that understanding the various causes of shell color polymorphism in marine gastropods is of great importance not only to understand how biodiversity works, but also for protecting such biodiversity, as knowledge of its evolutionary causes may help implement conservation measures in those species or ecosystems that are threatened.

Quantifying the Geometric Shell Shape between Populations of True Limpets Lottia Mesoleuca (Mollusca: Lottidae) in Colombia

The increasing activity in morphological studies has provided new tools to analyses the shape quantitatively, these quantitative measurements allow the researcher to examine the variation in shape and perform analysis to examine the quantitative differences among the species shapes, where geometric morphometrics has rendered great results in the last years. This study was focused on assessing the morphometric variation between populations of Lottia mesoleuca of the family Lottidae, an abundant group of gastropods in the rocky ecosystems of Bahía Málaga and Isla Gorgona (Colombian Pacific). This family has a high morphological diversity, making the identification of some morphotypes problematic work. Geometric morphometrics methods were applied on the shell using dorsal, lateral and ventral views. Different multivariate analyses were performed to differentiate the groups of species and populations (principal component analysis, morphological distances comparisons and grouping analysis by means of the Ward method). The results indicate that individuals of the species Lottia mesoleuca have key geometric characteristics associated to the different populations (depth intertidal zones) for classification, being the geometric shape of the shell enough to determine morphotypes between the different populations studied. Aspects associated with the combination of ecological variables with morphometric ones are necessary to be able to visualize with a higher resolution the structural complexity of populations and their adaptation processes. Furthermore, it is obvious that there is a strong need to conduct more explorations of environmental and ecological processes that provide some insight on why the morphological characteristics are so variable in the same species.

The Mendelian inheritance of rare flesh and shell colour variants in the black-lipped pearl oyster (Pinctada margaritifera)

Pinctada margaritifera is French Polynesia's most economically important aquaculture species. This pearl oyster has the specific ability to produce cultured pearls with a very wide range of colours, depending on the colour phenotypes of donor oysters used. Its aquaculture is still based on natural spat collection from wild stocks. We investigated three rare colour variants of P. margaritifera - orange flesh, and red and white shell colour phenotypes - in comparison with the wild-type black flesh and shell commonly found in this species. The study aimed to assess the geographic distribution and genetic basis of these colour variants. Colour frequencies were evaluated during transfer and graft processes of pearl oyster seed captured at collector stations. Among the collection locations studied, Mangareva Island showed the highest rate of the orange flesh phenotype, whereas Takaroa and Takume atolls had relatively high rates of red and white shell phenotypes respectively. Broodstocks were made of these rare colour variants, and crosses were performed to produce first- and second-generation progenies to investigate segregation. The results were consistent with Mendelian ratios and suggest a distinct model with no co-dominance: (i) a two-allele model for flesh trait, whereby the orange allele is recessive to the black fleshed type, and (ii) a three-allele model for shell trait, whereby the black wild-type allele is dominant to the red coloration, which is dominant to the white shell. Furthermore, the proposed model provides the basis for producing selected donor pearl oyster lines through hatchery propagation.

Identification of Single-Locus PCR-Based Markers Linked to Shell Background Color in the Pacific Oyster (Crassostrea gigas)

A number of Pacific oyster (Crassostrea gigas) with golden shell background color were obtained which show great potential to develop a niche market. To improve the selective breeding progress of true-breeding strains with complete golden oysters, research was conducted to identify genetic markers linked to the shell color locus. An F1-segregating population was obtained by crossing two oysters with golden shell and white shell. Genomic DNA from eight progenies with golden shell and eight progenies with white shell were equally pooled for amplified fragment length polymorphism (AFLP) screening. In bulked segregant analysis, six out of 225 selective primer pair combinations produced seven polymorphic fragments tightly linked to shell color across the segregating population. The seven AFLP markers were all derived from the golden dam and mapped onto a single linkage group flanking the shell color locus. In conversion of the AFLPs into single-locus PCR-based markers, a sequence-characterized amplified region (SCAR) marker, named SCARJ8-2, a single nucleotide polymorphism (SNP) marker, named SNPL2-4, and a simple sequence repeat (SSR) marker, named SSRO11-2, were obtained. These markers obtained in this study will be useful for marker-assisted selection of the Pacific oyster.

A consensus microsatellite-based linkage map for the hermaphroditic bay scallop (Argopecten irradians) and its application in size-related QTL analysis

Bay scallop (Argopecten irradians) is one of the most economically important aquaculture species in China. In this study, we constructed a consensus microsatellite-based genetic linkage map with a mapping panel containing two hybrid backcross-like families involving two subspecies of bay scallop, A. i. irradians and A. i. concentricus. One hundred sixty-one microsatellite and one phenotypic (shell color) markers were mapped to 16 linkage groups (LGs), which corresponds to the haploid chromosome number of bay scallop. The sex-specific map was 779.2 cM and 781.6 cM long in female and male, respectively, whereas the sex-averaged map spanned 849.3 cM. The average resolution of integrated map was 5.9 cM/locus and the estimated coverage was 81.3%. The proportion of distorted markers occurred more in the hybrid parents, suggesting that the segregation distortion was possibly resulted from heterospecific interaction between genomes of two subspecies of bay scallop. The overall female-to-male recombination rate was 1.13:1 across all linked markers in common to both parents, and considerable differences in recombination also existed among different parents in both families. Four size-related traits, including shell length (SL), shell height (SH), shell width (SW) and total weight (TW) were measured for quantitative trait loci (QTL) analysis. Three significant and six suggestive QTL were detected on five LGs. Among the three significant QTL, two (qSW-10 and qTW-10, controlling SW and TW, respectively) were mapped on the same region near marker AiAD121 on LG10 and explained 20.5% and 27.7% of the phenotypic variance, while the third (qSH-7, controlling SH) was located on LG7 and accounted for 15.8% of the phenotypic variance. Six suggestive QTL were detected on four different LGs. The linkage map and size-related QTL obtained in this study may facilitate marker-assisted selection (MAS) in bay scallop.

Water-borne stimuli released by predatory crabs and damaged prey induce more predator-resistant shells in a marine gastropod

Individuals of the morphologically variable, rocky intertidal gastropod Thais (or Nucella) lamellosa developed larger apertural teeth when held in the presence of the predatory crab Cancer productus than when held in its absence, regardless of whether snails were fed or not. In addition, among fed snails larger apertural teeth were produced in the presence of crabs fed conspecific snails than in the presence of crabs fed frozen fish. Because all snails were held in containers through which water flowed from physically separated aquaria holding the crabs, these results indicate that water-soluble chemical cues released by this predatory crab and by damaged conspecifics induced T. lamellosa to improve the defense effectiveness of their shells. Finally, when allowed access to food, snails exposed to these stimuli ate fewer barnacles and grew less than those in the controls.

Identification and mapping of amplified fragment length polymorphism markers linked to shell color in bay scallop, Argopecten irradians irradians (Lamarck, 1819)

Amplified fragment length polymorphisms (AFLP) were used to study the inheritance of shell color in Argopecten irradians. Two scallops, one with orange and the other with white shells, were used as parents to produce four F(1) families by selfing and outcrossing. Eighty-eight progeny, 37 orange and 51 white, were randomly selected from one of the families for segregation and mapping analysis with AFLP and microsatellite markers. Twenty-five AFLP primer pairs were screened, yielding 1138 fragments, among which 148 (13.0%) were polymorphic in two parents and segregated in progeny. Six AFLP markers showed significant (P < 0.05) association with shell color. All six loci were mapped to one linkage group. One of the markers, F1f335, is completely linked to the gene for orange shell, which we designated as Orange1, without any recombination in the progeny we sampled. The marker was amplified in the orange parent and all orange progeny, but absent in the white parent and all the white progeny. The close linkage between F1f335 and Orange1 was validated using bulk segregation analysis in two natural populations, and all our data indicate that F1f335 is specific for the shell color gene, Orange1. The genomic mapping of a shell color gene in bay scallop improves our understanding of shell color inheritance and may contribute to the breeding of molluscs with desired shell colors.

Inheritance of the general shell color in the scallop Argopecten purpuratus (Bivalvia: Pectinidae)

Although some external coloration and pigmentation patterns in molluscan shells may be attributable to environmental factors, most variation in these phenotypic characters depends on uncomplicated genetic mechanisms. Genetic research on inheritance of color variations in the north-Chilean scallop (Argopecten purpuratus) has now been expanded to analyze color segregation in juvenile scallops produced under controlled conditions employing self- and cross-fertilization. Calculations from the results were used for comparison with different numerical models based on Mendelian inheritance, and results were also obtained on the inheritance of a dorsoventral white line often observed on the left (upper) valve in this species. The results confirmed the hereditary basis for color variation in the shell of this scallop, suggesting a simple, dominant model of epistasis to explain the distribution of the different color variants observed (purple, brown, orange, yellow, and white). The presence of the white line may be controlled by a recessive allele with simple Mendelian traits on a locus distinct from those that control color variation.

Inheritance of allozymes of a marine snail (Urosalpinx cinerea)

Crosses using virgin oyster drills (Urosalpinx cinerea) show that both the ODH and LAP loci of foot muscle tissue have two co-dominant alleles segregating in Mendelian fashion. This constitutes the first demonstration of allozyme inheritance in a marine gastropod.