Gene conversion yields novel gene combinations in paralogs of GOT1 in the copepod Tigriopus californicus

Christopher S Willett

doi:10.1186/1471-2148-13-148

Abstract

BackgroundGene conversion of duplicated genes can slow the divergence of paralogous copies over time but can also result in other interesting evolutionary patterns. Islands of genetic divergence that persist in the face of gene conversion can point to gene regions undergoing selection for new functions. Novel combinations of genetic variation that differ greatly from the original sequence can result from the transfer of genetic variation between paralogous genes by rare gene conversion events. Genetically divergent populations of the copepod Tigriopus californicus provide an excellent model to look at the patterns of divergence among paralogs across multiple independent evolutionary lineages.ResultsIn this study the evolution of a set of paralogous genes encoding putative aspartate transaminase proteins (called GOT1 here) are examined in populations of the copepod T. californicus. One pair of duplicated genes, GOT1p1 and GOT1p2, has regions of high divergence between the copies in the face of apparent on-going gene conversion. The GOT1p2 gene also has unique haplotypes in two populations that appear to have resulted from a transfer of genetic variation via inter-paralog gene conversion. A second pair of duplicated genes GOT1Sr and GOT1Sd also shows evidence of gene conversion, but this gene conversion does not appear to have maintained each as a functional copy in all populations.ConclusionsThe patterns of conservation and sequence divergence across this set of paralogous genes among populations of T. californicus suggest that some interesting evolutionary patterns are occurring at these loci. The results for the GOT1p1/GOT1p2 paralogs illustrate how gene conversion can factor in the creation of a mosaic pattern of regions of high divergence and low divergence. When coupled with rare gene conversion events of divergent regions, this pattern can result in the formation of novel proteins differing substantially from either original protein. The evolutionary patterns across these paralogs show how gene conversion can both constrain and facilitate diversification of genetic sequences.

Highlights

Gene conversion of duplicated genes can slow the divergence of paralogous copies over time but can result in other interesting evolutionary patterns
Identification of GOT paralogs Five new paralogous genes were identified from a PCRbased screen of expressed sequences from the copepod T. californicus that are homologous to genes encoding aspartate transaminase proteins
These genes were sequenced in four populations including three from southern California, San Diego (SD), La Jolla (LJS), and Abalone Cove (AB), and one from central California, Santa Cruz (SCN)

Summary

Introduction

Gene conversion of duplicated genes can slow the divergence of paralogous copies over time but can result in other interesting evolutionary patterns. Concerted evolution can result from loci undergoing repeated gene conversion, which causes duplicated genes to evolve in tandem and not diverge from one another over evolutionary time. For gene duplicates undergoing some level of concerted evolution, divergence between them will not begin to increase markedly until a threshold of sequence divergence is breached (as high as 20 percent [5]) Models of this process suggest that there will typically be a long period of evolution with only low levels of divergence until a threshold level of divergence is passed at which point the rate of divergence will increase [6]. Using this method in a study in yeast, Takuno and Innan [8] identified two sets of duplicated heat shock proteins that likely fit this model

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