Steven W. Rissing

Indirect effects of granivory by harvester ants: plant species composition and reproductive increase near ant nests

SummaryOf 36 plant species surveyed, 6 were significantly associated with nests of the desert seed-harvester ant Veromessor pergandei or Pogonomyrmex rugosus; two other plant species were significantly absent from ant nests. Seeds of two common desert annuals, Schismus arabicus and Plantago insularis, realize a 15.6 and 6.5 fold increase (respectively) in number of fruits or seeds produced per plant growing in ant nest refuse piles compared to nearby controls. Mass of individual S. arabicus seed produced by plants growing in refuse piles also increased significantly. Schismus arabicus, P. insularis and other plants associated with ant nests do not have seeds with obvious appendages attractive to ants. Dispersal and reproductive increase of such seeds may represent a relatively primitive form of ant-plant dispersal devoid of seed morphological specializations. Alternatively, evolution of specialized seed structures for dispersal may be precluded by the assemblage of North American seed-harvester ants whose workers are significantly larger than those ants normally associated with elaiosome-attached seed dispersal. Large worker size may permit consumption of elaiosome and seed.

Extreme genetic differences between queens and workers in hybridizing Pogonomyrmex harvester ants

The process of reproductive caste determination in eusocial insect colonies is generally understood to be mediated by environmental, rather than genetic factors. We present data demonstrating unexpected genetic differences between reproductive castes in a variant of the rough harvester ant, Pogonomyrmex rugosus var. fuscatus. Across multiple loci, queens were consistently more homozygous than expected, while workers were more heterozygous. Adult colony queens were divided into two highly divergent genetic groups, indicating the presence of two cryptic species, rather than a single population. The observed genetic differences between castes reflect differential representation of heterospecific and conspecific patrilines in these offspring groups. All workers were hybrids; by contrast, winged queens were nearly all pure–species. The complete lack of pure–species workers indicates a loss of worker potential in pure–species female offspring. Hybrids appear to be bipotential, but do not normally develop into reproductives because they are displaced by pure–species females in the reproductive pool. Genetic differences between reproductive castes are expected to be rare in non–hybridizing populations, but within hybrid zones they may be evolutionarily stable and thus much more likely to occur.

Queen aggression pleometrotic advantage and brood raiding in the ant veromessor pergandei hymenoptera formicidae

Co-founding Veromessor pergandei queens tended brood equally throughout colony foundation. With the appearance of foragers, however, queens began to fight, resulting in an increase in queen death rate. Worker aggression towards queens occurred only after queen fighting began and was directed only to queens damaged during fights. Starting V. pergandei colonies raided brood from nearby nests; workers exhibited nest fidelity during such raids. Colonies started by multiple queens opened 9 days earlier than those started by single queens and were more successful at brood raiding. Queens and workers of defeated, brood-raided colonies abandoned their nests and joined/invaded victor nests. Brood raiding in this and similar species may be a consequence of territoriality and the clumped distribution of starting nests. Colony foundation by multiple queens, pleometrosis, may be an evolutionary response to brood raiding.

Worker size variability and foraging efficiency in Veromessor pergandei (Hymenoptera: Formicidae)

SummaryAs foragers of the harvester ant, Veromessor pergandei, travel further from their nest they spend significantly more time sampling seeds in experimental patches. Although accepted seeds are heavier than offered seeds, mass of accepted seed is not correlated with sampling time. Variably sized V. pergandei workers do not “size-match”; little, if any, variance in size of seed selected can be attributed to body size of forager. The lack of size-matching in V. pergandei suggests individual performance may be an inadequate measure of colony foraging success.

Loss of Phenotypic Plasticity Generates Genotype-Caste Association in Harvester Ants

Caste differentiation and reproductive division of labor are the hallmarks of insect societies. In ants and other social Hymenoptera, development of female larvae into queens or workers generally results from environmentally induced differences in gene expression. However, several cases in which certain gene combinations may determine reproductive status have been described in bees and ants. We investigated experimentally whether genotype directly influences caste determination in two populations of Pogonomyrmex harvester ants in which genotype-caste associations have been observed. Each population contains two genetic lineages. Queens are polyandrous and mate with males of both lineages , but in mature colonies, over 95% of daughter queens have a pure-lineage genome, whereas all workers are of F1 interlineage ancestry. We found that this pattern is maintained throughout the colony life cycle, even when only a single caste is being produced. Through controlled crosses, we demonstrate that pure-lineage eggs fail to develop into workers even when interlineage brood are not present. Thus, environmental caste determination in these individuals appears to have been lost in favor of a hardwired genetic mechanism. Our results reveal that genetic control of reproductive fate can persist without loss of the eusocial caste structure.

Genetic Relatedness Among Co-Foundresses of Two Desert Ants, Veromessor Pergandei and Acromyrmex Versicolor(Hymenoptera: Formicidae)

Cooperative colony foundation occurs in some social hymenoptera. Polistine wasp foundress associations are usually composed of close relatives (reviewed in Gamboa et al. 1986, Michener and Smith 1987) suggesting kin selection may play an important role in establishment of such groups. Cooperative colony foundation, however, may be advantageous even if cofoundresses are not related (Lin and Michener 1972, Pollock and Rissing 1988a). Indeed, several behavioral (reviewed in Kissing and Pollock 1988) and one electrophoretic (Ross and Fletcher 1985) study suggest ant foundress associations form without respect to relatedness. Here we report on an electrophoretic analysis of intra-group relatedness among co-foundresses of Veromessor pergandei and Acromyrmex versicolor, two common desert ants with cooperative colony foundation (Pollock and Rissing 1985, Rissing and Pollock 1986, Rissing et al. 1986). Ideally, relatedness should be measured directly through pedigree analysis of interacting individuals (Hamilton 1972). Since this is impractical for most natural populations of social insects, the alternative is indirect estimation using neutral genetic markers (Pamilo and Crozier 1982, Pamilo 1984). We used polymorphic allozyme loci, detected by protein electrophoresis, for this purpose (Richardson et al. 1986). Allozyme loci offer the advantage that homozygous and heterozygous individuals are readily distinguishable; in addition, these loci are not likely involved directly in determining behavior patterns and thus can be treated as selectively neutral within the context of social evolution (Pamilo 1984).

Annual cycles in worker size of the seed-harvester antVeromessor pergandei (Hymenoptera: Formicidae)

SummaryThe seed-harvester antVeromessor pergandei Mayr is primitively polymorphic; workers are monophasically allometric. There is a distinct annual cycle in mean worker body size that replicates across colonies and habitats (Fig. 1); this cycle occurs through alteration of the worker size distribution (Fig. 2). There is little, if any, morphspecific task specialization by workers suggesting worker size variance is a colony-level adaptation permitting maintenance of a large and constant worker force during periods of resource fluctuation. Smaller workers appear in the foraging force following the “triple crunch” of reduced seed availability, reduced favorable times to forage, and alate production during winter months. Adult and startingV. pergandei colonies exhibit strong intraspecific territoriality, suggesting the selective advantage for maintenance of a large and constant worker force. Such selective pressures may have provided the initial variance in worker size distributions that led subsequently to specialized castespecific task performance in more distinctly polymorphic ant species.

Social interaction among pleometrotic queens of Veromessor pergandei (Hymenoptera: Formicidae) during colony foundation

Queens of the pleometrotic seed-harvester ant Veromessor pergandei associate without respect to relatedness during colony foundation in laboratory choice tests. This species may be used as a test case for social competition where kin selection cannot occur. Co-founding queens contribute equally to the initial brood and exist without dominance prior to eclosion of their first workers. Before worker eclosion, claustral founding ants exhibit a closed-energy system. We argue that social competition cannot operate under such circumstances. Among pleometrotic, claustral founding ants, inter-queen aggression should not occur before worker emergence: this is verified for V. pergandei.

MOLECULAR EVIDENCE FOR THE ORIGIN OF WORKERLESS SOCIAL PARASITES IN THE ANT GENUS POGONOMYRMEX

Abstract.— Speciation of two social parasites from their respective hosts is tested using a molecular phylogeny. Alignment of 711 DNA base pairs of mitochondrial cytochrome b gene was used to assess phylogenetic relationships of inquiline species to their hosts and to other members of the genus. We show that the inquiline social parasites of the North American seed harvester ants are monophyletic, descending from one of the known hosts (Pogonomyrmex barbatus) in the recent past and shifting hosts in a pattern similar to that observed in other Hymenopteran social parasites. In addition, the host populations unexpectedly were found to be polyphyletic. Populations of Pogonomyrmex rugosus from an area east of the Chiricahua Mountains in Southern Arizona belong to a mitochondrial clade separate from the more western clade of P. rugosus from the Sonoran and Chihuahuan Deserts. Evidence of mitochondrial DNA introgression between P. rugosus and P. barbatus was also observed. We conclude that Emerys rule does not strictly hold for this system, but that the hosts and parasites are very closely related, supporting a loose definition of Emerys rule.

Resource abundance and sex allocation by queen and workers in the harvester ant, Messor pergandei

Abstract. The degree to which queens and workers control how limiting resources are split between reproductive males and females is central to the study of sex allocation in the eusocial Hymenoptera. We investigated the effect of resource availability on sex allocation decisions by both queens and workers in the ant Messor pergandei. We conducted the following field manipulations of resource availability: food supplementation while queens were laying reproductive brood (early-fed treatment), removal of workers while queens were laying reproductive brood (worker-removal treatment), food supplementation of colonies while workers were tending reproductive brood (late-fed treatment), and unmanipulated colonies (control). Early-fed colonies produced more alates and exhibited more strongly female-biased sex ratios than other treatments. Worker-removal colonies produced the fewest alates and the least female-biased sex ratios. Late-fed colonies yielded individual alates with the heaviest fresh masses (males and females) and dry masses (only females). Aside from worker-removal colonies, the sex and investment ratios of colonies in this study were significantly more female-biased than the relatedness asymmetries hypothesis under worker control would predict. Consistent with the multifaceted parental investment hypothesis, the timing of food supplementation relative to the reproductive cycle of the colony plays a prominent role in influencing sex allocation by both queens and workers. Early food supplements resulted in increased number of females, whereas late food supplements resulted in heavier individual females. Temporal dynamics of food availability may explain part of the tremendous inter-population variation in colony sex ratios seen in this and other ant species. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00265-002-0462-6.