Patriline shifting leads to apparent genetic caste determination in harvester ants

Abstract

The harvester ant, Pogonomyrmex occidentalis, is characterized by high levels of intracolonial genetic diversity resulting from multiple mating by the queen. Within reproductively mature colonies, the relative frequency of different male genotypes (patrilines) is not stable. The difference between samples increases with time, nearing an asymptote after a year. Patriline distributions in gynes and workers display similar patterns of change. A consequence of changing patriline distributions is that workers and gynes appear to have different fathers. However, apparent genetic differences between castes are caused by changing paternity among all females. Temporal variation in the relative frequency of patrilines may be a consequence of processes that reflect sexual conflict, such as sperm clumping. Recent work documenting genotype differences between physical castes (workers and gynes; major and minor workers) in several species of ants has been interpreted as evidence of genetic caste determination. Reanalysis of these studies found little support for this hypothesis. Apparent caste determination may result from temporal variation in sperm use, rather than from fertilization bias among male ejaculates. M orphological traits frequently display phenotypic plasticity (reviewed in refs. 1, 2); such variation is especially common in insects (3). Phenotypic plasticity generally is assumed to be responsible for physical castes in the social insects, including differences between gynes (virgin queens) and workers (nonreproductive caste) and differences between workers (major and minor worker castes). The proximal cause of the plasticity is thought to be largely nutritional (4, 5), and in some species, such as the honey bee, the underlying nutritional mechanisms are well understood (6, 7). As with any phenotype, physical caste is unlikely to be completely environmentally determined, and genotypes may differ in their propensity to develop into gynes or workers under a given set of environmental conditions (4, 8). Historically, genetic variation among individuals has not been thought to contribute to caste determination, although a number of scattered publications have suggested a role for genetic differences, especially in the ants (9-13).However, several recent studies have reported genetic determination of caste in ants. Reproductive females in some species of ants are produced clonally, whereas workers are produced sexually [e.g., Cataglyphis cursor (14); Wasmannia auropunctata (15)]. Some populations of harvester ants (Pogonomyrmex) comprise two independent but interacting genetic lineages. The genome of reproductive females in such populations is composed of a single lineage, whereas females that contain genes from both lineages become workers (16)(17)(18). A similar phenomenon occurs in hybridizing populations of fire ant species (19). In these species, there is a clear genetic effect.A second set of studies involves species with polyandrous queens that find differences in the distribution of male genotypes between gyn...

Variation in patriline reproductive success during queen production in orphaned colonies of the thelytokous antCataglyphis cursor

“…2003; Moritz et al. 2005; Acromyrmex echinatior , Hughes & Boomsma 2008; Pogonomyrmex occidentalis Wiernasz & Cole 2010), but not in others (e.g. Formica species , Keller et al.…”

“…2002; Johnson et al. 2009; Wiernasz & Cole 2010). Cheating can ultimately lead to some kind of social parasitism if the cheating genotypes frequently drift and reproduce in foreign colonies.…”

Beyond promiscuity: mate-choice commitments in social breeding

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Public goods dilemma in asexual ant societies

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