Marine invertebrates and fish are well known for their remarkable genetic diversity, which is commonly explained by large population size and the characteristic dispersive nature of their early, planktonic life histor...Marine invertebrates and fish are well known for their remarkable genetic diversity, which is commonly explained by large population size and the characteristic dispersive nature of their early, planktonic life history. Other potential sources of diversity in marine animals, such as a higher mutation rate, have been much less considered, though evidence for a high genetic load in marine bivalves has been accumulating for nearly half a century. In this review, I examine evidence for a higher genetic load in marine animals from studies of molecular marker segregation and linkage over the last 40 years, and survey recent work examining mutational load with molecular evolution approaches. Overall, marine animals appear to have higher genetic load than terrestrial animals （higher dn/ds ratios, inbreeding load, and segregation dis＇tortion）, though results are mixed for marine fish and data are lacking for many marine animal groups. Bivalves （oysters） have the highest loads observed among marine animals, comparable only to long-lived plants; however, more data is needed from other bivalves and more marine invertebrate taxa generally. For oysters, a higher load may be related to a chronically lower effective population size that, in concert with a higher mutational rate, elevate the number of deleterious mutations observed. I suggest that future studies use high-throughput sequencing approaches to examine （1） polymorphism in genomescale datasets across a wider range of marine animals at the population level and （2） intergenerational mutational changes between parents and offspring in crosses of aquaculture species to quantify mutation rates.展开更多
Major histocompatibility complex （MHC） is a family of highly polymorphic genes activating adaptive immunity in vertebrates. However, the underlying mecha- nism of MHC evolution is still not fully understood. In this...Major histocompatibility complex （MHC） is a family of highly polymorphic genes activating adaptive immunity in vertebrates. However, the underlying mecha- nism of MHC evolution is still not fully understood. In this study, we investigated genetic variation of three classical MHC class I genes in the giant panda （Ailuropoda mela- noleuca） and tested for selection effect and recombination event across exonic and intronic sequences to understand maintenance mechanism of polymorphism at Aime-MHC class I genes. In total, we isolated 21 MHC class I haplotypes （exon 2-intron 2-exon 3） from 46 captive giant pandas, of which eight were for Aime-C, seven for Aime-I and six for Aime-L; however, we only identified six unique sequences from these haplotypes. The subsequent maximum-likeli- hood and Chi-square analyses both detected evidence of recombination acting on the 21 haplotypes. These results indicate that the giant panda still retains a relatively high adaptive variation at Aime-MHC-I genes, and that the intronic segments have been homogenized along evolu- tionary time by recombination and subsequent genetic drift.We calculated nucleotide substitution rates of the antigen- binding regions （exons 2 and 3） and the noncoding intron 2, and found two pieces of evidence supporting the presence of balancing selection in the giant panda： an excess of nonsynonymous over synonymous substitutions at the antigen-binding sites, and an obviously higher synonymous substitutions in the exons than nucleotide substitutions in the intron. Thus, this study reveals that balancing selection and recombination together shape the diversity pattern at Aime- MHC-I loci of the giant panda.展开更多
文摘Marine invertebrates and fish are well known for their remarkable genetic diversity, which is commonly explained by large population size and the characteristic dispersive nature of their early, planktonic life history. Other potential sources of diversity in marine animals, such as a higher mutation rate, have been much less considered, though evidence for a high genetic load in marine bivalves has been accumulating for nearly half a century. In this review, I examine evidence for a higher genetic load in marine animals from studies of molecular marker segregation and linkage over the last 40 years, and survey recent work examining mutational load with molecular evolution approaches. Overall, marine animals appear to have higher genetic load than terrestrial animals （higher dn/ds ratios, inbreeding load, and segregation dis＇tortion）, though results are mixed for marine fish and data are lacking for many marine animal groups. Bivalves （oysters） have the highest loads observed among marine animals, comparable only to long-lived plants; however, more data is needed from other bivalves and more marine invertebrate taxa generally. For oysters, a higher load may be related to a chronically lower effective population size that, in concert with a higher mutational rate, elevate the number of deleterious mutations observed. I suggest that future studies use high-throughput sequencing approaches to examine （1） polymorphism in genomescale datasets across a wider range of marine animals at the population level and （2） intergenerational mutational changes between parents and offspring in crosses of aquaculture species to quantify mutation rates.
基金supported by a special grant(SG1411)for the giant panda from the State Forestry Administration of China
文摘Major histocompatibility complex （MHC） is a family of highly polymorphic genes activating adaptive immunity in vertebrates. However, the underlying mecha- nism of MHC evolution is still not fully understood. In this study, we investigated genetic variation of three classical MHC class I genes in the giant panda （Ailuropoda mela- noleuca） and tested for selection effect and recombination event across exonic and intronic sequences to understand maintenance mechanism of polymorphism at Aime-MHC class I genes. In total, we isolated 21 MHC class I haplotypes （exon 2-intron 2-exon 3） from 46 captive giant pandas, of which eight were for Aime-C, seven for Aime-I and six for Aime-L; however, we only identified six unique sequences from these haplotypes. The subsequent maximum-likeli- hood and Chi-square analyses both detected evidence of recombination acting on the 21 haplotypes. These results indicate that the giant panda still retains a relatively high adaptive variation at Aime-MHC-I genes, and that the intronic segments have been homogenized along evolu- tionary time by recombination and subsequent genetic drift.We calculated nucleotide substitution rates of the antigen- binding regions （exons 2 and 3） and the noncoding intron 2, and found two pieces of evidence supporting the presence of balancing selection in the giant panda： an excess of nonsynonymous over synonymous substitutions at the antigen-binding sites, and an obviously higher synonymous substitutions in the exons than nucleotide substitutions in the intron. Thus, this study reveals that balancing selection and recombination together shape the diversity pattern at Aime- MHC-I loci of the giant panda.
