Salomon, Matthew et al. (2009) International Worm Meeting "Persistence time of mutations affecting fitness and body size in Caenorhabditis briggsae and C. elegans."

Upadhyay, Ambuj, Salomon, Matthew, Baer, Charles, Levy, Laura, Keller, Thomas, Phillips, Naomi, Blanton, Dustin, Ostrow, Dejerianne, Bour, Whitney, Sylvestre, Thamar

[International Worm Meeting, 2009]

The level of genetic variation present in a population is a composite function of mutation, population size, and natural selection. Historically, efforts to understand differences (or similarities) between groups in levels of genetic variation have focused on the interplay between population size and natural selection. However, much less attention has been paid to the alternative possibility that differences among groups are due to systematic differences in the underlying rate of mutation. Much of the difficulty in interpreting the role of mutation stems from the fact that most of what is known about genomic mutational properties, for quantitative traits in multicellular eukaryotes, comes from a handful of phylogenetically distant and biologically dissimilar model organisms, making meaningful comparisons difficult. Over the past several years our lab has been investigating the properties of new mutations in a model nematode system within a comparative phylogenetic framework. Mutations have been allowed to accumulate in the (relative) absence of natural selection, thus allowing us to estimate the genetic variance introduced by new mutation (VM) for two species of rhabditid nematodes, Caenorhabditis elegans and C. briggsae. Previous work in this system suggests that the mutation rate in C. briggsae is on the order of twice that of C. elegans for quantitative traits and dinucleotide repeats. Here we report the standing genetic variance (VG) for two quantitative traits, lifetime reproduction and body size, in worldwide collections of C. briggsae and C. elegans natural isolates. Comparisons of VG to VM between the natural isolates and our mutation accumulation lines allow us to infer the magnitude and pattern of constraint on phenotypic evolution in these two species. Taking the results from the two species together, the persistence time (VG/VM) of new mutations affecting fitness is on the order of tens to perhaps hundreds of generations, with an average selection coefficient against homozygotes of a few per-cent. Furthermore, the pattern of persistence time for mutations affecting adult body size is onsistent with that of fitness in both species. These results suggest that idiosyncratic selection, perhaps due to random hitchhiking - "genetic draft" - is paramount in shaping the standing genetic variance of these traits in these species.