Intron characterization and their potential as molecular markers for population studies in the scallopsAequipecten opercularisandMimachlamys varia

The genome sequence of the variegated scallop, Mimachlamys varia (Linnaeus, 1758)

We present a genome assembly from an individual Mimachlamys varia (the variegated scallop; Mollusca; Bivalvia; Pectinida; Pectinidae). The genome sequence is 975.4 megabases in span. Most of the assembly is scaffolded into 19 chromosomal pseudomolecules. The mitochondrial genome has also been assembled and is 21.78 kilobases in length.

Characterization of testis-specific serine/threonine kinase 1-like (TSSK1-like) gene and expression patterns in diploid and triploid Pacific abalone (Haliotis discus hannai; Gastropoda; Mollusca) males

Testis-specific serine/threonine kinase 1-like (TSSK1-like), which plays important roles in late-phase spermatogenesis and male fertility, was characterized in Pacific abalone Haliotis discus hannai, an important commercial marine gastropod. Further, its expression patterns were assessed in diploid and induced triploid males showing differential degrees of testis maturation. Abalone TSSK1-like shared conserved structural features with mammalian TSSK1s and other potential metazoan orthologs, especially regarding the catalytic STKc domain. Phylogenetically, abalone TSSK1-like displayed a genetic affiliation with its molluscan TSSK1-like orthologs and human TSSK1. Additionally, abalone TSSK1-like gene showed a tetrapartite exon-intron organization, unlike the intronless structure of most amniotic tetrapodian TSSK1s. Molecular phylogenetic analysis in the metazoan lineage suggested a possible revision in the origin of the earliest ancestral TSSK1. Further, abalone TSSK1-like showed testis-predominant expression, which was significantly influenced by both age and seasonal reproductive cycles. Comparative expression analyses between diploid and triploid abalone males suggested that robust TSSK1-like expression occurred primarily at the post-meiotic stage. Additionally, RT-PCR assay indicates that mature abalone sperms retain TSSK1-like transcripts after release. Taken together, this study provides useful insights for further studies to assess male reproduction and sterility and/or partial fertility of induced male triploidy in abalone species.

Inferring Invasion History of Red Swamp Crayfish (Procambarus clarkii) in China from Mitochondrial Control Region and Nuclear Intron Sequences

Identifying the dispersal pathways of an invasive species is useful for adopting the appropriate strategies to prevent and control its spread. However, these processes are exceedingly complex. So, it is necessary to apply new technology and collect representative samples for analysis. This study used Approximate Bayesian Computation (ABC) in combination with traditional genetic tools to examine extensive sample data and historical records to infer the invasion history of the red swamp crayfish, Procambarus clarkii, in China. The sequences of the mitochondrial control region and the proPOx intron in the nuclear genome of samples from 37 sites (35 in China and one each in Japan and the USA) were analyzed. The results of combined scenarios testing and historical records revealed a much more complex invasion history in China than previously believed. P. clarkii was most likely originally introduced into China from Japan from an unsampled source, and the species then expanded its range primarily into the middle and lower reaches and, to a lesser extent, into the upper reaches of the Changjiang River in China. No transfer was observed from the upper reaches to the middle and lower reaches of the Changjiang River. Human-mediated jump dispersal was an important dispersal pathway for P. clarkii. The results provide a better understanding of the evolutionary scenarios involved in the rapid invasion of P. clarkii in China.

Quantitative genetic analysis of Cry1Ab tolerance in Ostrinia nubilalis Spanish populations

Tolerance to Bacillus thuringiensis Cry1Ab toxin in Spanish Ostrinia nubilalis populations was analyzed by quantitative genetic techniques, using isolines established from field-derived insects. F1 offspring was tested for susceptibility to trypsin activated Cry1Ab using a concentration that caused a mean larval mortality of 87% (±17% SD). The progeny of the most tolerant isolines (that had shown mortalities lower than 60%) was crossed to obtain the F2 generation that was exposed to the same Cry1Ab concentration. A clear reduction in mortality (62±17% SD) was observed. The upper limit for heritability was estimated to range between 0.82 and 0.90, suggesting that a high part of phenotypic variation in tolerance to Cry1Ab was attributable to genetic differences. An estimate of the minimum number of segregating factors indicated that the loci involved in tolerance to Cry1Ab were at least two. The role of the cadherin gene, which is a B. thuringiensis resistance gene in Lepidoptera, was assessed in the most tolerant isolines by using an EPIC-PCR marker specifically developed for this study. Association between cadherin and tolerance was obtained in one tolerant isoline; however it could be not confirmed by segregation analysis in the F2 progeny because F2 offspring was not viable. Our results indicate that the tolerance trait is common in Spanish field populations. Quantitative genetic techniques may be helpful for estimating the influence of genetic factors to Cry1Ab tolerance in O. nubilalis.

A high load of non-neutral amino-acid polymorphisms explains high protein diversity despite moderate effective population size in a marine bivalve with sweepstakes reproduction

Marine bivalves show among the greatest allozyme diversity ever reported in Eukaryotes, putting them historically at the heart of the neutralist-selectionist controversy on the maintenance of genetic variation. Although it is now acknowledged that this high diversity is most probably a simple consequence of a large population size, convincing support for this explanation would require a rigorous assessment of the silent nucleotide diversity in natural populations of marine bivalves, which has not yet been done. This study investigated DNA sequence polymorphism in a set of 37 nuclear loci in wild samples of the flat oyster Ostrea edulis. Silent diversity was found to be only moderate (0.7%), and there was no departure from demographic equilibrium under the Wright-Fisher model, suggesting that the effective population size might not be as large as might have been expected. In accordance with allozyme heterozygosity, nonsynonymous diversity was comparatively very high (0.3%), so that the nonsynonymous to silent diversity ratio reached a value rarely observed in any other organism. We estimated that one-quarter of amino acid-changing mutations behave as neutral in O. edulis, and as many as one-third are sufficiently weakly selected to segregate at low frequency in the polymorphism. Finally, we inferred that one oyster is expected to carry more than 4800 non-neutral alleles (or 4.2 cM(-1)). We conclude that a high load of segregating non-neutral amino-acid polymorphisms contributes to high protein diversity in O. edulis. The high fecundity of marine bivalves together with an unpredictable and highly variable success of reproduction and recruitment (sweepstakes reproduction) might produce a greater decoupling between Ne and N than in other organisms with lower fecundities, and we suggest this could explain why a higher segregating load could be maintained for a given silent mutation effective size.