Doctoral Dissertation Improvement: Ecological Correlates of Differential Selection on S Opsin Genes in Nocturnal Primates

University Of Texas At Austin, Austin TX

Investigators

Abstract

Most mammals possess two types of retinal cone photoreceptors for color discrimination: (1) short-wavelength-sensitive (S) cones that are most responsive to blues and violets, and (2) medium/long-wavelength-sensitive (M/L) cones that are most responsive to greens and yellows. However, recent research has revealed that the S opsin gene, which codes for the production of S cones, has mutated and lost functionality in several groups of nocturnal primates (lorises, galagos, and owl monkeys). By contrast, the S opsin gene has remained functional in other nocturnal primate groups (tarsiers and most lemurs). Nocturnal non-primate mammals exhibit similar variation in the presence or absence of S cones. Currently the ecological factors involved in the retention or loss of the S cone in primates and other mammals are unclear. This research will examine the influence of variation in ambient light on evolutionary pressures for nocturnal primates to retain or lose their S cones. More specifically, this research will test the prediction that primates inhabiting nocturnal environments rich in short-wavelength light will experience increased selection to maintain a functional S opsin gene. Conversely, primates inhabiting nocturnal environments impoverished in short-wavelength light are expected to experience relaxed selection to maintain S opsin gene functionality. To achieve this goal, the S opsin gene will be sequenced in a large sample of nocturnal lemurs from five genera (Avahi, Cheirogaleus, Lepilemur, Microcebus, and Phaner). Species of these genera are endemic to one of three distinct habitat types in Madagascar (spiny forests, dry deciduous forests, and rainforests) that are expected to vary in the availability of short-wavelength light based on differences in foliage density. The recent evolutionary history of the S opsin gene will be directly examined for all species using statistical tests that can discriminate between purifying selection (i.e., selection to maintain the function of a gene) and relaxed selection to maintain gene functionality. The sample size and breadth (152 individuals from 20 species) will permit both within-species and between-species selection tests. The type and strength of selection acting on the S opsin gene will be compared between congeneric species from different habitat types in order to understand the effects of habitat light on the evolution of primate color vision. Identifying the selective factors involved in the retention or loss of S cones is fundamental to understanding why only some nocturnal primates and have lost their S cones while others have retained them. There is also currently debate regarding which activity pattern characterized the last common ancestor of living primates. By testing an adaptive explanation for S cone retention in nocturnal species, this project will help to resolve disagreements about the activity patterns of basal primates and is thus important for evaluating current adaptive hypotheses of primate origins. Additionally, identifying an adaptive framework for S cone loss would permit paleoenvironmental reconstructions for primate and mammalian lineages based on variation in S opsin expression. Finally, this project will make publicly available the largest sample of nocturnal primate S opsin gene sequences yet assembled and provide educational experience and training for both graduate and undergraduate students.