Microarrays and species origins

摘要

Whole-genome arrays have been used to reveal small islands of genetic differentiation in Anopheles mosquitoes. Analysis of these regions will identify genes involved in the initial stages of speciation. What are the genetic modifications that underliethe first steps in the origin of new species? The use of microarrays to make whole-genome comparisons between populations that can interbreed, but only to a limited degree, provides a fresh approach to the question. As reported in PLoS Biology, Turner et al. have applied this methodology to the malaria-carrying mosquito, Anopheles gam-biae. They have identified three small regions, less than 1,5% of the genome, that contain critical genes for the initiation of speciation. In the classic model of speciation, two populations diverge during a period of geographical separation and cannot exchange genes when the physical barrier is removed. In this case the two genomes accumulate differences uniformly. There is, however, often gene flow during the period of divergence, or after secondary contact if reproductive isolation is incomplete. In these cases, genetic differentiation becomes unevenly distributed across the genome as gene flow erodes differences between species, leaving islands of differentiated sequence only around the genes responsible for reproductive isolation. This pattern has been nicely illustrated by the sequencing of multiple genetic loci in closely related species pairs of the fruitfly Drosophila, and by surveying differentiation at many anonymous markers, for example in the winkle, Littorina saxatilis.