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Large-scale comparison of sequence polymorphism and divergence at numerous genomic loci within and between closely related species can reveal signatures of natural selection. Here, we present a population genomics study based on direct sequencing of 61 mitotic cell cycle genes from 30 Arabidopsis thaliana accessions and comparison of the resulting data to the close relative Arabidopsis lyrata. We found that the Arabidopsis core cell cycle (CCC) machinery is not highly constrained but is subject to different modes of selection. We found patterns of purifying selection for the cyclin-dependent kinase (CDK), CDK subunit, retinoblastoma, and WEE1 gene families. Other CCC gene families often showed a mix of one or two constrained genes and relaxed purifying selection on the other genes. We found several large effect mutations in CDKB1;2 that segregate in the species. We found a strong signature of adaptive protein evolution in the Kip-related protein KRP6 and departures from equilibrium at CDKD;1 and CYCA3;3 consistent with the operation of selection in these gene regions. Our data suggest that within Arabidopsis, the genetic robustness of cell cycle-related processes is more due to functional redundancy than high selective constraint. [ABSTRACT FROM AUTHOR]

We report on the construction of sex-specific high-density linkage maps and identification of sex-linked markers for the black tiger shrimp (Penaeus monodon). Overall, we identified 44 male and 43 female linkage groups (2n = 88) from the analysis of 2306 AFLP markers segregating in three full-sib families, covering 2378 and 2362 cM, respectively. Twenty-one putatively homologous linkage groups, including the sex-linkage groups, were identified between the female and male linkage maps. Six sex-linked AFLP marker alleles were inherited from female parents in the three families, suggesting that the P. monodon adopts a WZ-ZZ sex-determining system. Two sex-linked AFLP markers, one of which we converted into an allele-specific assay, confirmed their association with sex in a panel of 52 genetically unrelated animals. [ABSTRACT FROM AUTHOR]