World Status, Exploitation and Trade - WIDECAST

......INTRODUCTIONand that gene flow is sufficiently high to maintain a rather homogenous genepool (Bonhomme et al , 1987). Although an earlier and more restrictedsurvey of 13 loci (Smith et al . 1977) demonstrated moderately highheterozygosity in C. mydas . the significance of this is uncertain becausehalf of the sample was composed of a mixture of animals from eggs laid incaptivity at Cayman Turtle Farm, derived from three different sources in theCaribbean.Bowen et al. (1989) report results of a recent study of mitochondrial DNAsequences in C. mydas samples from four localities (Hawaii, Florida,Venezuela, Ascension). The study revealed that the three Atlantic coloniesare very closely related, with all samples sharing at least 93 of 95restriction sites. Three mtDNA genotypes were found, each of the threeAtlantic samples were characterised by a pattern not observed in othersamples, but the three genotypes differed only by one or two mutationsteps. This does suggest, as stated by Bowen et al , "a contemporaryrestriction of female-mediated gene flow between Atlantic rookeries", butthe overall pattern is of close similarity between localities rather than ofmarked difference. The Hawaiian sample was readily distinguished fromAtlantic samples by five restriction enzymes, with a total difference of sixmutation steps.In summary, none of the work reported to date has provided experimentalevidence of near-complete genetic isolation of geographically isolatednesting aggregations. What is known is that in all cases for which goodtag-return data exist, nesting females, when they nest in more than oneseason, typically nest again on the beach on which they first nested. Whatis widely suspected, but not yet demonstrated, is that mature males andfemales return to their natal beach, and that each nesting colony is aseparate and closed reproductive unit. Experimental investigation of thenatal beach hypothesis will require marking of hatchlings usingsophisticated techniques similar to those used in the study of migratoryfish stocks.The study of mitochondrial DNA sequences in C. mydas by Bowen et al (1989)revealed that the Ascension sample is closely allied to other Atlanticsamples. Ascension Island has been isolated for more than 40 million yearsas a result of sea-floor spreading, and genetic isolation of the AscensionC. mydas population for this period of time should, even allowing for areduced mutation rate owing to long generation time, have resulted inradical changes in nucleotide sequence. This was not observed, and Bowenet al conclude that the Ascension Island population, or extensive gene flowinto it, has been of recent origin.More general discussion of their mtDNA sequence results led Bowen et al(1989) to suggest that natal homing operates on a regional basis, ratherthat being specific to one nesting beach, and that a Green Turtle"population" consists of clusters of neighbouring nesting colonies, ratherthan being equivalent to a single nesting colony. Gyuris and Limpus (1988)recently reported very low electrophoretic variability in Caretta frommainland and island nest sites in east Australia, despite an extremely lowinterchange of individual turtles between sites. They attributed this inpart to multiple paternity (Harry and Briscoe, 1988) and the probabilitythat the genetic input of males is distributed after mating to both mainlandand Capricornia Reef rookery sites. Again, the basic "population" consistsof a cluster of neighbouring nesting colonies.20