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Submitted to: Journal of Nematology
Publication Type: Abstract Only Publication Acceptance Date: 6/16/2013 Publication Date: 12/4/2013 Citation: Timper, P. 2013. Negative frequency-dependent selection between Pasteuria penetrans and its host Meloidogyne arenaria. Journal of Nematology. 45:321. Interpretive Summary: Technical Abstract: In negative frequency-dependant selection (NFDS), parasite genotypes capable of infecting the numerically dominant host genotype are favored, while host genotypes resistant to the dominant parasite genotype are favored, creating a cyclical pattern of resistant genotypes in the host population and, after a brief lag, virulent genotypes in the parasite population. The net effect is a selection against the numerically dominant host and parasite genotypes over time. For NFDS to occur there must be multiple host and parasite genotypes in the population, genotypic specificity for parasitism, and a high fitness cost to the host from parasitism. The bacterial parasite Pasteuria penetrans substantially reduces egg production of its nematode host, Meloidogyne spp. and endospores of the bacterium have a high degree of specificity for attachment to second-stage juveniles (J2). The objective of this study was to determine the changes in endospore attachment to J2 over time in a field population of P. penetrans that was parasitizing M. arenaria. Once per year from 1998 to 2012, soil was collected from four replicate plots at the Gibbs Farm near Tifton, GA. Juveniles from a greenhouse (GH) culture of M. arenaria were added to soil from each plot to bioassay for endospore attachment and, starting in 2010, J2 from a single egg mass line (SEM 6) were also used in the bioassay. From 1998 to 2006, endospore acquisition progressively declined from 95% to 2% of the GH J2 with spores. From 2007 to 2010, endospore acquisition increased from 12% to 87% of the GH J2 with spores. In recent years, endospore acquisition by GH J2 has again started to decline from 67% in 2011 to 53% in 2012, whereas endospore acquisition by SEM 6 J2 was 97 to 100% from 2010 to 2012. Moreover, average spores per J2 increased from 4 to 22 for the SEM 6 line and decreased from 13 to 8 for the GH culture between 2010 and 2011. The GH culture, like the field population of M. arenaria, is heterogeneous for endospore attachment; however, the GH culture is not under selection pressure from P. penetrans. Consequently, genotypic frequencies in the GH culture should remain constant. The changes in spore acquisition over time likely reflect changes in the frequency of P. penetrans genotype capable of adhering to the dominant host genotype in the GH culture and to the single genotype in SEM 6. It appears that the dominant attachment genotype in the GH culture is different from SEM 6. |
