Genetic structure and diversity in New World screwworm populations from Uruguay revealed by microsatellite markers: Implications for control strategies

Full text: The New World screwworm (NWS), Cochliomyia hominivorax, is one of the most important parasitic insect pests that causes invasive myiasis in warm-blooded vertebrates and is responsible for important economic losses to livestock rearing throughout the neotropics. Because of the substantial economic losses caused by this pest, an international effort has eradicated the NWS from endemic areas of North and Central America and is preventing invasions into screwworm-free areas. There have been speculations and conflicting reports, however, about the existence of non-interbreeding populations and their possible effects on the control programme. It is crucial to identify such populations and to characterise the genetic variability and structure of target populations in order to maximize the effectiveness of the eradication programme. To investigate the genetic variability and population structure across the current geographic distribution of the NWS, we focus on microsatellites markers, which due to their high polymorphism are ideal for addressing such questions. The recent isolation and characterization of polymorphic microsatellite markers for C. hominivorax now allows the investigation of the genetic variability and population structure of this pest in Uruguay. Five microsatellite markers previously isolated were used in this study to characterise C. hominivorax samples from five distinct geographic locations in Uruguay. All five loci investigated were polymorphic, revealing a high degree of polymorphism across the five sampling locations. For the five microsatellite loci analysed, the expected heterozygosity varied from 0.33 to 0.84 and the observed heterozygosities varied from 0.19 to 0.79. The number of alleles detected per locus was high, varying from 7 to 11. Significant deviation from Hardy-Weinberg equilibrium was recorded for four sampling localities. In all cases, departures from H-W expectations were due to an excess of homozygotes. Among the possible factors that might account for the observed deviations is the occurrence of demographic changes that affected NWS populations. Two main reasons could be responsible for such changes: The decrease in temperature and humidity in the Uruguayan winter and persistent insecticide treatments. The observed homozygous excess may also be a result of the Wahlund effect, since screwworms from different farms at each location were sampled. However, such effects should be apparent in most of the loci across populations, which was not the case for this data set. The presence of null alleles, therefore, may be the main responsible for the observed deviations. The five populations exhibited remarkably similar allele distributions and shared the same predominant alleles at four of the examined loci. Two measures of interpopulation genetic differentiation were used in this study (FST and RST). The global multilocus estimate of RST was 0.026 and of FST was 0.029. Both estimates, although low, were numerically very similar and significantly different from zero (P 0.02630, for RST and P =0.001, for FST), suggesting that little differentiation exists among these populations. The number of migrants per generation (Nm) between localities based on the estimated FST was 6.38 and based on RST was 9.19, indicating that there is a large amount of migration even between geographically distant populations. FST estimates between populations ranged from -0.0037 to 0.0712 and were, for five of the ten population pairs, significantly different from zero at the 0.05 level. The analysed populations showed no isolation by distance (r2 = 0.007, P = 0.53), suggesting that simple migration models may not be sufficient to explain the low differentiation between NWS populations. One factor that cannot be neglected for this pest is the possibility of passive migration of larvae by the movement of infested animals. An alternative explanation can be considered as responsible for the low differentiation and the lack of isolation by distance. The absence of isolation by distance could be indicative of a recent recolonisation event. Considering the hypothesis of mass-mortality, by either climatic conditions or insecticide treatment, a recolonisation by a large founder population could cause a demographic turnover if this population rapidly spreads over Uruguay during climatically favourable seasons. Using microsatellite markers, we have provided some baseline data of genetic variation to which other NWS populations can be compared. Further sampling of other populations throughout its geographical distribution would extend this study and determine if similar patterns of genetic variation and gene flow are observed. Such information will provide valuable guidelines for control strategies of this livestock pest. (author)