Changes in Mating System and Social Structure of the Ant Petalomyrmex Phylax Are Associated With Range Expansion in Cameroon

Abstract

Past climate shifts have led to major oscillations in species distributions. Hence historical contingencies and selective processes occurring during such phases may be determinants for understanding the forces that have shaped extant phenotypes. In the plant-ant Petalomyrmex phylax (Formicinae), we observed spatial variation in number of queens in mature colonies, from several queens (high polygyny) in the median part of its distribution to a moderate number of queens (weak polygyny) or even only a single queen (monogyny) in the southwesternmost populations. This variation did not correlate with indicators of variation in current nest site availability and colony turnover, the supposedly determinant selective forces acting on gyny in ants. We show here that the variation in social structure correlates with a historical process corresponding to a progressive colonization of coastal southern Cameroon by the ant. Using microsatellite markers, we observed a clear pattern of isolation by distance except for the southernmost populations. Measures of genetic variability that do not take into account allele size were at equilibrium in all except the southernmost populations, suggesting recent foundation of the latter. Measures of genetic diversity taking into account allele size showed a clinal north-south decrease in variance of allele size. We propose that southern populations have yet to regain allele size variance after bottlenecks associated with the foundation of new populations, and that this variance is regained over time. Hence variation in social structure mirrors an old but still active southward colonization process or metapopulation dynamics, possibly in association with an expansion of the rain forest habitat during the late Holocene. A low number of queens in ant colonies is typically associated with strong dispersal capacity. We therefore suggest that the initial founders of new populations belong to the monogynous to weakly polygynous phenotype, and that queen number progressively increases in older populations. Phenotypic variation was first considered in an adaptationist perspective, emphasizing the role of natural selection. Consistent with this view, many studies suggest that ongoing selection explains a large part of observed variability in phenotypic traits (e.g., Merilä 1997; Petit et al. 2001;González-Martínez et al. 2002;Steinger et al. 2002). However, it has also been shown that the result of current selection can be constrained by other evolutionary processes, such as strong gene flow or genetic drift (Lynch et al. 1999;Hendry 2002;Lenormand 2002;Magiafoglou et al. 2002). Hence, it is now widely accepted that phenotypic variation has to be explained by taking into account not only ongoing selection, but also historical contingencies (Thompson et al. 1998;Davison and Clarke 2000) and neutral processes (Bonnin et al. 1996;Hardy et al. 2000).Ants present extensive variation in social structure of colonies and queen reproductive strategies, traits that have clear adaptive implications. However, t...

Range Expansion Drives Dispersal Evolution In An Equatorial Three-Species Symbiosis

“…(ii) Genetic variation in C. mckeyi was analyzed for 206 diploid individuals from 14 populations (mean = 15 individuals per population, range = 5–62, each from a different colony) sampled in 2000–2002, and genotypic data from 10 markers were already available for this sample [54]. (iii) Genetic variation in P. phylax was analyzed for 475 diploid individuals from 21 populations (mean = 23 individuals per population, range = 13–66, each from a different colony) sampled in 2000–2002, and genotypic data from 12 markers were already available for this sample [55]. Within species, all genetic markers were shown to be statistically independent with the program GENEPOP 3.3 [56].…”

“…The two last rows indicate the means of values over all populations and the correlation between population means and the map distance from the southernmost limit of the range (Spearman rank correlation coefficient rS, asterisks indicate significant correlation [*: P<0.05, **: P<0.01, ***: P<0.001]). Data for P phylax are from Dalecky et al [55].…”

Host Plant Use by Competing Acacia-Ants: Mutualists Monopolize While Parasites Share Hosts

“…Our genetic analyses showed relatedness values among workers from P. ferrugineus colonies around 0.75 and allele counts that indicated monogyny and an effective mating frequency of 1.09 (one out of eleven queens likely mated with two males). High relatedness among workers seems to be common in mutualistic plant-ant systems as described for Pseudomyrmex peperi , which is an extremely polygynous acacia mutualist in Central America [30] as well as for the mutualistic Petalomyrmex phylax , which inhabits Leonardoxa africana in Cameroon [42]. Some Myrmelachista species have large polygynous colonies that occupy monospecific patches of understory Duroia and Tococa ant-plants forming so-called “devil's gardens” [43].…”

Changes in composition of cuticular biochemicals of the facultatively polygynous ant Petalomyrmex phylax during range expansion in Cameroon with respect to social, spatial and genetic variation

“…Populations are patchily distributed and generally form dense and well‐defined aggregates of several hundreds of colonies. Population genetic analyses along a north–south transect ending at the southwestern edge of the species distribution have shown that P. phylax has experienced a southward colonization that proceeded by stepwise colonization with founder effects (Dalecky et al . 2007).…”

“…This suggests that the local recovery of genetic diversity at microsatellite loci partly results from accumulation of local mutations. Dalecky et al . (2007) proposed that this ant species has experienced a southward colonization process in association with an expansion of its rainforest habitat since the late Holocene.…”