Bryant McAllister - US grants

0075295
McAllister

A morphologically and functionally distinct pair of sex chromosomes provides a basis for gender determination in many organisms. One chromosome (X) is shared between the two genders, and contains a normal set of functional genes; the other (Y) is restricted to one gender, and has few genes. These differences arise through a repeatable evolutionary process, because pairs of sex chromosomes have arisen multiple times from pairs of identical chromosomes (autosomes), a transition that has been demonstrated for the X-Y pair in humans.

This project involves molecular and genetic analyses of Drosophila americana americana, a species with a chromosomal rearrangement that has recently transformed a pair of its autosomes into neo-sex chromosomes. The long-term objective of the study is to reveal the forces influencing loss of gene function on the male-limited neo-Y chromosome of D. a. americana. Studies will characterize the pattern of sequence variation in the region adjacent to the centromere of these neo-sex chromosomes, because this region is subjected to a gradient of restricted recombination. These data will lead to a better understanding of the relationship between recombination and the forces of mutation, selection and genetic drift.

The study of Y chromosomes provides a model system for demonstrating the consequences of restricted recombination, which has implications for understanding factors influencing asexual reproduction and evolution within closely-linked regions of the genome.

Genomes of organisms are subdivided into chromosomes. Comparisons among closely related species generally reveal differences in chromosomal organization as a result of rearrangements, yet chromosomal rearrangements in humans are associated with birth defects and cancers. The consequences of naturally occurring chromosomal rearrangements and the importance of genome reorganization are essentially unknown. Rearranged chromosomes within populations represent ideal systems for determining effects of chromosomal form on organismal function. A latitudinal cline for a chromosomal rearrangement in the fly Drosophila americana is an unusually tractable system for examining the significance of chromosomal form. This project will identify DNA sequence variation associated with alternative chromosomal forms, determine the influence of the rearrangements on the maintenance of this variation, and reveal differences among individuals owing to chromosomal form. These data will test the hypothesis that the alternative chromosomal arrangements in D. americana coordinate adaptive variation along this north-south climatic gradient.

Scientific training of students will be facilitated at multiple levels during the project. Two graduate students will be directly involved in data collection and analysis. Analyses of frequencies of the alternative chromosomal forms in natural populations of D. americana will be incorporated as an exercise in an undergraduate teaching laboratory. Students will gain unparalleled hands-on experience by performing and interpreting genetic analyses of genome organization.

The cold conditions of winter 2009-10 throughout the southern US offer a rare opportunity to assess the immediate biological impacts of extreme climate fluctuation. This project will document the effects of the extremely cold winter on Drosophila americana, a fly species native to North America and distributed over a broad latitudinal range. Previous studies of this species reveal polymorphisms in genes and traits that allow persistence in time of extreme temperatures, and these genotypes and phenotypes differentiate the extreme northern and southern populations. This study will assess genotypic and phenotypic variation in southern populations to test whether polymorphisms characteristic of northern populations have a higher prevalence immediately following an extremely cold winter.

Many features of the environment vary predictably in space and time, such as seasonal climate gradients in temperate regions. Organisms adapt to local conditions, yet day-to-day and year-to-year fluctuations occur as a consequence of weather systems. Genetic variation within species may play an important role in buffering populations against these fluctuations. This project will measure the immediate changes in the genetic and adaptive trait composition of populations following an extreme climatic event. The project will contribute to improved understanding of the capacity for genetic variation to buffer against climatic extremes, which is an important aspect of adaptation and persistence of species persistence as global climate and weather change.