Abstract:
Two tansy-feeding aphids, Macrosiphoniella tanacetaria (MA) and Metopeurum fuscoviride (ME), were studied at a
small spatial scale in and around Jena (< 80 km2) using polymorphic microsatellite markers. Both species were found
in approximately 60% of sites formerly known to harbour the aphids, although, generally when they did occur, they
occurred singly (MA ~50%; ME ~60%) and rarely together on the same plant at the same time (approximately 10%)
and then usually only in the early part of the growing season. This difference may be a result of quasi-apparent
competition effects elicited by ants farming ME aphids, and preferentially actively eliminating or disturbing MA
aphids. In terms of population genetics, both aphids showed extreme genetic heterogeneity within a metapopulation
structure, with ME more than MA (i.e. higher FST values, approximately 0.4 versus 0.15, respectively), and limited
levels of interpopulation gene flow. Subpopulations often deviated from Hardy–Weinberg equilibrium and showed
linkage disequilibria, as expected in animals with extended parthenogenetic reproduction, and had positive FIS values
for most large samples, suggesting inbreeding, and possibly philopatry, certainly in ME. Hierarchical analysis (allele
range and number per locus, analysis of molecular variance and FST) strongly suggested that the plant rather than
site governs the level of genetic variation. Bayesian clustering analysis revealed that both species had heterogeneous
historical genetic patterning, with K (number of subgroups) in the range 3–7. Evidence is also provided from
isolation-by-distance and private allele analyses indicating that, in MA, the presence of winged autumnmales, absent
in ME where males are wingless, influences comparative population genetic structuring, such that ME subpopulations
are comparatively more inbred and genetically differentiated than MA subpopulations. Lastly, additional
spatial arrangement (ALLELES-IN-SPACE) analysis showed that, in both species, certain subpopulations were
genetically isolated from the remainder, probably as a result of geographical barriers, including intervening buildings
and woods. As such, the biology of these tansy aphids living in semi-natural habitats is very different from many pest
aphid species examined within agro-ecosystems and infesting ephemeral crops. This is because the former appear to
be much more reluctant to fly and hence show contrastingly much higher levels of interpopulation divergence, even
at small spatial scales as investigated in the present study. Indeed, the number of genotypic clusters found for tansy
aphids using Bayesian approaches is similar to that globally for the major pest, the peach-potato aphid, Myzus
persicae.
