Conservation and Dynamics of Microsatellite Loci over 300 Million Years of Marine Turtle Evolution
Nancy N. FitzSimmons, * Craig Moritz, * and Stephen S. Moor@
*Department of Zoology, and jCommonwealth Scientific and Industrial Research Organisation, Molecular Animal Genetics Core, University of Queensland
Microsatellite loci consisting of (CA), repetitive arrays were obtained from threespecies of marine turtle, and primers were designed to test for polymorphism within species and the persistence of microsatellites across species. Homologous loci were found in each test of six marine species within two families (Cheloniidae and Dermochelyidae), as well as in a freshwater species (Emydidae, Trachemys scripta), which indicates a conservation of flanking sequences spanningapproximately 300 million years of divergent evolution. The persistence of homologous microsatellites across marine turtles was confirmed by direct sequencing of loci across species and by the discovery of polymorphism in 24 of 30 cross species tests. The conservation of flanking sequences could be due to a slow rate of base substitution in turtle nuclear DNA, as previously reported for mtDNA. In contrast,the presence of up to 25 alleles per locus per species indicates that the replication slippage events responsible for changes in allele length operate as in mammals. Comparisons of alleles among species revealed that alleles of the same length may not be homologous due to mutations within the flanking sequences. Levels of heterozygosity were consistently higher in species from which the primers weredesigned, which suggests problems with cross-species comparisons of variability. Within species, microsatellite variation between divergent populations was consistent with results from previous mtDNA studies indicating the usefulness of microsatellites for comparing male- versus femalemediated gene flow. Introduction
Microsatellite loci, sequences containing arrays of short (2-5 bp) tandemrepeats with variable copy number (Tautz 1989), have proved to be powerful tools for gene mapping ( Weber and May 1989; Weissenbach et al. 1992) and paternity studies (reviewed in Queller et al. 1993 ) because of their high heterozygosity. It has also been suggested that microsatellite loci have great potential for broader applications such as comparative gene mapping (Beckman and Soller 1990; Mooreet al. 199 1) and assessing genetic population structure within species (Bowcock et al. 1994; Roy et al. 1994). For this potential to be realized, we need a better understanding of how microsatellite loci evolve within and across species (Bruford and Wayne 1993; Valdes .et al. 1993). On the one hand, it is possible that mutation rates are so high that common ancestry is obscured even withinpopulations. On the other hand, there may be constraints on microsatellite loci resulting in excessive convergence of
Key words: microsatellite, population genetics,evolutionary rates, DNA polymorphism, Testudines, dinucleotide repeats, marine turtles. Address for correspondence and reprints: Nancy N. FitzSimmons, Department of Zoology, University of Queensland, Brisbane, Queensland, 4072, Australia.E-mail: N.Fitzsimmons@mailbox.uq.oz.au Mol. Biol. Evol.
12(3):432-440. 1995. 0 1995 by The University of Chicago. All rights reserved. 0737-4038/95/ 1203-0008$02.00
allelic states, leading to underestimation of allelic divergence among populations. Information on the dynamics of microsatellite evolution has come mainly from studies of intraspecific polymorphism, predominantly in humans.Mutation rates estimated from pedigrees are in the range of 10 -210m5(Edwards et al. 1992; Mahatani and Willard 1993), and changes in copy number of the repeats are thought to be due to slipped strand mispairing (Schlotterer and Tautz 1992). Mutations observed within pedigrees typically involve the gain or loss of one or a few repeat units (reviewed by Valdes et al. 1993, but see Kuhl and Caskey 1993 )...
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