Randy Smalls Research Interests
Plant Molecular Systematics
Molecular evolutionary studies seek to understand the processes by which DNA sequences change and to infer historical genetic events from the patterns observed in contemporary data. In this field, my research interests are in the evolutionary dynamics of nuclear gene families, and in patterns of nucleotide diversity in nuclear-encoded genes.
Almost all nuclear genes exist in gene families - groups of genes with a common evolutionary origin that (usually) have similar functions. These gene families vary from just a few to hundreds of copies per genome. One of the central questions is why does gene copy number vary so considerably both across gene families and across species? New genes must be "born" (via gene duplication) and old genes must "die" (become pseudogenized or deleted) for such variation in copy number to occur, but as yet we have little empirical information on the frequency or underlying causes of these phenomena. I am using the phylogenetically well-characterized cotton genus Gossypium> to examine rates and patterns of gene family evolution, with particular attention to the alcohol dehydrogenase (Adh) gene family.
I am also interested in levels and patterns of genetic diversity at the DNA sequence level ("nucleotide diversity"). Levels of nucleotide diversity can be used to infer what molecular evolutionary processes have been acting on a particular sequence, and to compare genes both within a given genome, as well as between species. Genes that are highly conserved are likely to be under strong purifying selection that maintains a particular gene function. At the other end of the spectrum, genes (especially duplicated genes) may undergo positive Darwinian selection resulting a new function or expression pattern. Comparisons of orthologous genes between species may provide clues as to how those species differ; comparisons of paralogous genes within species may provide clues as to how that particular gene family have evolved.
Finally, the phenomenon of polyploidy throws an interesting twist into studies of molecular evolution because whole genomes are immediately duplicated providing complete genetic redundancy and thus allowing for the possibility of divergence in function of the duplicated copies. The cotton genus (Gossypium) provides an excellent model system for studying polyploidy induced molecular evolutionary changes because it contains both allotetraploid species and good models of their diploid progenitors. Thus the null hypothesis of additivity and evolutionary stasis in the polyploid can be examined by comparing sequences of the diploid progenitors to sequences from the polyploid. To test this null hypothesis using diploid and tetraploid Gossypium as a model, I am continuing to explore the Adh gene family, and studies have also been initiated on the GBSSI gene family, and a group of floral homeotic MADS-box genes.
Molecular Evolution Papers