The Nature of Selection on the Major Histocompatibility Complex.

Only natural selection can account for the extreme genetic diversity of genes of the major histocompatibility complex (MHC). Although the structure and function of classic MHC genes is well understood at the molecular and cellular levels, there is controversy about how MHC diversity is selectively maintained. The diversifying selection can be driven by pathogen interactions and inbreeding avoidance mechanisms. Pathogen-driven selection can maintain MHC polymorphism based on heterozygote advantage or frequency-dependent selection due to pathogen evasion of MHC-dependent immune recognition. Empirical evidence demonstrates that specific MHC haplotypes are resistant to certain infectious agents, while susceptible to others. These data are consistent with both heterozygote advantage and frequency-dependent models. Additional research is needed to discriminate between these mechanisms. Infectious agents can precipitate autoimmunity and can potentially contribute to MHC diversity through molecular mimicry and by favoring immunodominance. MHC-dependent abortion and mate choice, based on olfaction, can also maintain MHC diversity and probably functions both to avoid genome-wide inbreeding and produce MHC-heterozygous offspring with increased immune responsiveness. Although this diverse set of hypotheses are often treated as competing alternatives, we believe that they all fit into a coherent, internally consistent thesis. It is likely that at least in some species, all of these mechanisms operate, leading to the extreme diversification found in MHC genes.

Phylogenetic and ecological determinants of the neotropical dawn chorus.

The concentration of avian song at first light (i.e. the dawn chorus) is widely appreciated, but has an enigmatic functional significance. One widely accepted explanation is that birds are active at dawn, but light levels are not yet adequate for foraging. In forest communities, the onset to singing should thus be predictable from the species' foraging strata, which is ultimately related to ambient light level. To test this, we collected data from a tropical forest of Ecuador involving 57 species from 27 families of birds. Time of first song was a repeatable, species-specific trait, and the majority of resident birds, including non-passerines, sang in the dawn chorus. For passerine birds, foraging height was the best predictor of time of first song, with canopy birds singing earlier than birds foraging closer to the forest floor. A weak and opposite result was observed for non-passerines. For passerine birds, eye size also predicted time of first song, with larger eyed birds singing earlier, after controlling for body mass, taxonomic group and foraging height. This is the first comparative study of the dawn chorus in the Neotropics, and it provides the first evidence for foraging strata as the primary determinant of scheduling participation in the dawn chorus of birds.

Serum immunoglobulin G levels are positively related to reproductive performance in a long-lived seabird, the common tern (Sterna hirundo).

The evolution of longevity requires a low risk of mortality from extrinsic factors, relative to intrinsic factors, so that individuals that differentially invest in physiological self-maintenance and minimize their annual reproductive costs will maximize lifetime fitness through a prolonged reproductive lifespan. The trade-off between reproductive effort and self-maintenance, as measured by immune function, has been well documented in short-lived birds, but is difficult to demonstrate in long-lived birds. To assess self-maintenance in a long-lived seabird, we measured serum protein levels, including immunoglobulin G (IgG = IgY), in 30 breeding pairs of common terns (Sterna hirundo) and their first-hatched (A) chicks. Most parents were of known age from banding as hatchlings; our sample was selected to contrast young breeders (6-9 years) with very old birds (17-23 years). Body-mass of the parents declined by 5% during the chick-rearing period, while serum protein levels were stable. Serum IgG levels were higher in parents of offspring with faster growth rates, while IgG levels were lower in parents whose broods were reduced by starvation. A-chicks in broods of two had higher IgG levels than singleton chicks. Albumin levels were not related to reproductive performance. Thus, despite adequate statistical power, we could find no evidence for a trade-off between reproduction and self-maintenance in common terns, even in old age. The results are consistent with life-history predictions for long-lived vertebrates, in which selection favors sustained self-maintenance across the reproductive lifespan. The positive relationships between IgG levels and reproductive performance indicate that IgG can be used as an index of parental "quality."

Restricting fluid intake during a single meal did not affect food intake in older adults.

Restricting fluid intake has been shown to decrease ad libitum food intake in animals and young adult humans. The purpose of this study was to determine if restricting fluid intake during a meal affects food intake in older adults. In a crossover counterbalanced design, 24 subjects (11 m, 13 f), 61-95 years, received lunch at a congregate meal site. Identical meals were accompanied by a volume of water equal to either 40% (restricted) or 100% (control) of each subject's usual lunch fluid intake. Pre-meal urine osmolality and specific gravity were used as indicators of hydration status. Weighed food intake was not different between the restricted and control conditions (400 g/2875 kJ and 408 g/2971 kJ, respectively). No significant correlations were found between urine osmolality or specific gravity and food intake, either in response to fluid restriction or per kg body weight. These results suggest that the appetite of healthy free-living older adults is not affected by fluid restriction during a single eating episode.

Ecological correlates of single-seededness in a woody tropical flora.

A reduction in seed number per fruit is recognized as a common evolutionary trend among flowering plants. In order to evaluate the ecological role of single-seededness and its possible adaptive significance, we used van Roosmalen's (1985) descriptions of fruits for woody species in the Guianan flora to identify fruit and seed characters and dispersal syndromes associated with the single-seeded trait. We coded the following characters: seed number (one seed or more than one seed), fleshiness (dry or fleshy), dehiscence (dehiscent or indehiscent), dispersal syndrome, seed size (length), and fruit size (length). We ordered the data in a multi-dimensional contingency table and used maximum likelihood logistic regression to identify traits significantly correlated with single-seededness. Seed size and fruit size were treated as quantitative variables. Indehiscence and endozoochory are positively associated with single-seededness, with indehiscence contributing most to the best-fit model. Fruit size and seed size are also important with the probability of single-seededness generally increasing with seed size and decreasing with fruit size, although a (fruit size) x (seed size) interaction term is significant. Dry fruits are positively associated with single-seededness and dispersal by synzoochory or myrmecochory negatively associated when the full data set is examined, but neither parameter is significant in two models constructed to remove effects of phylogeny. A nested ANOVA revealed that most variation occurs below the family level for almost all of the traits considered, with the exception of the dry vs. fleshy trait for which there is no variation within genera. We argue that the strong association between indehiscence and single-seededness suggests selective advantages for single-seeded dispersal units but acknowledge that energetic trade-offs between seed number and seed size probably also occur. We suggest that the post-dispersal fates of seeds - especially those deposited in clumps by endozoochory - should be examined with the idea of identifying selective pressures on seed number per fruit.