Contrasting Epidemic Histories Reveal Pathogen-Mediated Balancing Selection on Class II MHC Diversity in a Wild Songbird

The extent to which pathogens maintain the extraordinary polymorphism at vertebrate Major Histocompatibility Complex (MHC) genes via balancing selection has intrigued evolutionary biologists for over half a century, but direct tests remain challenging. Here we examine whether a well-characterized epidemic of Mycoplasmal conjunctivitis resulted in balancing selection on class II MHC in a wild songbird host, the house finch (Carpodacus mexicanus). First, we confirmed the potential for pathogen-mediated balancing selection by experimentally demonstrating that house finches with intermediate to high multi-locus MHC diversity are more resistant to challenge with Mycoplasma gallisepticum. Second, we documented sequence and diversity-based signatures of pathogen-mediated balancing selection at class II MHC in exposed host populations that were absent in unexposed, control populations across an equivalent time period. Multi-locus MHC diversity significantly increased in exposed host populations following the epidemic despite initial compromised diversity levels from a recent introduction bottleneck in the exposed host range. We did not observe equivalent changes in allelic diversity or heterozygosity across eight neutral microsatellite loci, suggesting that the observations reflect selection rather than neutral demographic processes. Our results indicate that a virulent pathogen can exert sufficient balancing selection on class II MHC to rescue compromised levels of genetic variation for host resistance in a recently bottlenecked population. These results provide evidence for Haldane's long-standing hypothesis that pathogens directly contribute to the maintenance of the tremendous levels of genetic variation detected in natural populations of vertebrates

Microalgal pigments: A source of natural food colors

Naturally sourced colorants and dyes are currently gaining demand over synthetic alternatives due to an increase in consumer awareness brought forward by health and environmental issues. Microalgae are unicellular organisms which are microscopic in size and represent major photosynthesizers with the ability to efficiently convert available solar energy to chemical energy. Due to their distinct advantages over terrestrial plants such as faster growth rates, ability to grow on non-arable land, and diversity in the production of various natural bioactive compounds (e.g., lipids, proteins, carbohydrate, and pigments), microalgae are currently gaining promise as a sustainable source for the production of natural food-grade colorants. The versatility of microalgae to produce various pigments (e.g., chlorophylls, carotenoids, xanthophylls, and phycobiliproteins) that can be commercially exploited as a source of natural colorant is there to be explored. Various growth factors such as temperature, pH, salinity, and light in terms of both quality and quantity have been shown to significantly impact pigment production. In this chapter, we comprehensively review the characteristics of microalgal pigments and factors that affect pigment production in microalgae while evaluating the overall feasibility of exploiting them as a natural source of food colorants