No abstract

Although many studies confirm long-term small isolated populations (e.g. island endemics) commonly sustain low neutral genetic variation as a result of genetic drift, it is less clear how selection on adaptive or detrimental genes interplay with random forces. We investigated sequence variation at two major histocompatibility complex (Mhc) class II loci on a porpoise endemic to the upper Gulf of California, México (Phocoena sinus, or vaquita). Its unique declining population is estimated around 500 individuals. Single-strand conformation polymorphism analysis revealed one putative functional allele fixed at the locus DQB (n = 25). At the DRB locus, we found two presumed functional alleles (n = 29), differing by a single nonsynonymous nucleotide substitution that could increase the stability at the dimer interface of alphabeta-heterodimers on heterozygous individuals. Identical trans-specific DQB1 and DRB1 alleles were identified between P. sinus and its closest relative, the Burmeister's porpoise (Phocoena spinipinnis). Comparison with studies on four island endemic mammals suggests fixation of one allele, due to genetic drift, commonly occurs at the DQA or DQB loci (effectively neutral). Similarly, deleterious alleles of small effect are also effectively neutral and can become fixed; a high frequency of anatomical malformations on vaquita gave empirical support to this prediction. In contrast, retention of low but functional polymorphism at the DRB locus was consistent with higher selection intensity. These observations indicated natural selection could maintain (and likely also purge) some crucial alleles even in the face of strong and prolonged genetic drift and inbreeding, suggesting long-term small populations should display low inbreeding depression. Low levels of Mhc variation warn about a high susceptibility to novel pathogens and diseases in vaquita.

Interannual changes in the occurrence of Bryde’s whale (Balaenoptera edeni) have been observed in La Paz Bay (southwestern Gulf of California, Mexico) over the last 20 years. We suggest that these changes could be driven by natural fluctuations in food resources that are related to climate variability. We compared monthly Bryde’s whale occurrence in La Paz Bay from 1988 to 2006 to climate variability at seasonal and interannual time scales and its effect on prey availability. The results showed that Bryde’s whales do not have a well-defined pattern of seasonal occurrence; however, large numbers of whales were consistently recorded during La Niña conditions when the Gulf of California sardine population is distributed further south within the gulf. In contrast, fewer whales were recorded during El Niño and neutral conditions. This indicates that changes in the occurrence of Bryde’s whales at La Paz Bay are driven by the El Niño-Southern Oscillation interannual variability and are probably mediated by their prey availability.

One of the most isolated populations of fin whales occurs in the Gulf of California (GOC) with 400-800 individuals. This population shows reduced neutral genetic variation in comparison to the North Pacific population and thus might also display limited adaptive polymorphism. We sampled 36 fin whales from the GOC and assessed genetic variation at exon 2 of the major histocompatibility complex class II DQB-1 genes responsible for eliciting immune responses. Three divergent alleles were found with higher nonsynonymous than synonymous substitution rates within the peptide-binding region positions as well as the likely retention of ancient alleles, indicating that positive selection has shaped diversity in this species. Limited levels of nonneutral polymorphism, in addition to previously described low levels of neutral polymorphism, are consistent with the results of previous studies on vertebrate populations that have remained small and demographically stable for a very long time. Such low genetic variation in the GOC fin whales could be explained by 2 demographic scenarios: an ancient isolated population with limited gene flow or a more recent founder event after the last glacial maximum with very restricted gene flow.

Sequences from exons encoding the peptide binding region of MHC class I (MHC-I) molecules were isolated from California gray whale (Eschrichtius robustus) genomic DNA to initiate an investigation of variation in these genes in a cetacean. These represent the first mysticete MHC-I sequences to be reported. The analysis of gray whale MHC-I sequences suggests the presence of at least three loci, which share greatest similarity to MHC-I in the ungulates, consistent with current views on cetacean phylogenetics. The peptide binding region of MHC is the most polymorphic part of the molecule and analysis of the variation and synonymous to nonsynonymous substitution ratios in gray whale sequences found these genes to display polymorphism characteristics similar to that attributed to selection in other species.