Low major histocompatibility complex class II diversity in European and North American moose.

Major histocompatibility complex (MHC) genes encode cell surface proteins whose function is to bind and present intracellularly processed peptides to T lymphocytes of the immune system. Extensive MHC diversity has been documented in many species and is maintained by some form of balancing selection. We report here that both European and North American populations of moose (Alces alces) exhibit very low levels of genetic diversity at an expressed MHC class II DRB locus. The observed polymorphism was restricted to six amino acid substitutions, all in the peptide binding site, and four of these were shared between continents. The data imply that the moose have lost MHC diversity in a population bottleneck, prior to the divergence of the Old and New World subspecies. Sequence analysis of mtDNA showed that the two subspecies diverged at least 100,000 years ago. Thus, viable moose populations with very restricted MHC diversity have been maintained for a long period of time. Both positive selection for polymorphism and intraexonic recombination have contributed to the generation of MHC diversity after the putative bottleneck.

High-precision 40Ar/39Ar geochronology and the advent of North America’s Late Cretaceous terrestrial fauna

A densely sampled, diverse new fauna from the uppermost Cedar Mountain Formation, Utah, indicates that the basic pattern of faunal composition for the Late Cretaceous of North America was already established by the Albian-Cenomanian boundary. Multiple, concordant 40Ar/39Ar determinations from a volcanic ash associated with the fauna have an average age of 98.39 ± 0.07 million years. The fauna of the Cedar Mountain Formation records the first global appearance of hadrosaurid dinosaurs, advanced lizard (e.g., Helodermatidae), and mammal (e.g., Marsupialia) groups, and the first North American appearance of other taxa such as tyrannosaurids, pachycephalosaurs, and snakes. Although the origin of many groups is unclear, combined biostratigraphic and phylogenetic evidence suggests an Old World, specifically Asian, origin for some of the taxa, an hypothesis that is consistent with existing evidence from tectonics and marine invertebrates. Large-bodied herbivores are mainly represented by low-level browsers, ornithopod dinosaurs, whose radiations have been hypothesized to be related to the initial diversification of angiosperm plants. Diversity at the largest body sizes (>106 g) is low, in contrast to both preceding and succeeding faunas; sauropods, which underwent demise in the Northern hemisphere coincident with the radiation of angiosperms, apparently went temporarily unreplaced by other megaherbivores. Morphologic and taxonomic diversity among small, omnivorous mammals, multituberculates, is also low. A later apparent increase in diversity occurred during the Campanian, coincident with the appearance of major fruit types among angiosperms, suggesting the possibility of adaptive response to new resources.

Forest losses predict bird extinctions in eastern North America.

Claims that there will be a massive loss of species as tropical forests are cleared are based on the relationship between habitat area and the number of species. Few studies calibrate extinction with habitat reduction. Critics raise doubts about this calibration, noting that there has been extensive clearing of the eastern North American forest, yet only 4 of its approximately 200 bird species have gone extinct. We analyze the distribution of bird species and the timing and extent of forest loss. The forest losses were not concurrent across the region. Based on the maximum extent of forest losses, our calculations predict fewer extinctions than the number observed. At most, there are 28 species of birds restricted to the region. Only these species would be at risk even if all the forests were cleared. Far from providing comfort to those who argue that the current rapid rate of tropical deforestation might cause fewer extinctions than often claimed, our results suggest that the losses may be worse. In contrast to eastern North America, small regions of tropical forest often hold hundreds of endemic bird species.