Presentation Title

How do populations of two hybridizing harvester ants coexist with one another?

Abstract

Interbreeding with another species is usually a costly mistake, as interspecific hybrids are often sterile. For social insects such as ants, however, hybrids can be advantageous if they develop into the worker caste, which do not directly reproduce. In the harvester ant genus Pogonomyrmex, queens of two different genetic lineages, J1 and J2, mate with and store sperm from males of both their own and the other lineage; hybrid females develop as sterile workers, while pure-lineage females develop as queens. In places where they interact, the proportion of each lineage is expected to equalize at 50:50 as selection acts against lineages that become too common, yet in the field the frequency of J1:J2 most often stabilizes around 40:60, suggesting that J2 colonies have an advantage. In this study, I tested four ecological advantage hypotheses to explain J2 dominance: enhanced foraging activity, colony size, territory size, and selective mating behavior. Worker activity and nest diameters were compared between 10 J1 colonies and 37 J2 colonies that were spatially mapped at a field site in Apache, Arizona. To assess mating behavior, 15 worker and 15 reproductive queen samples were collected from each colony and genotyped at 12 microsatellite markers, which were used to estimate the number of same and alternate-lineage males the queens had mated with. No significant differences were found for either worker activity or nest diameter between J1 and J2 colonies. The two lineages were randomly distributed across the site, with no difference in nearest neighbor distance between colony types. Research determining worker size and number of male mates of either lineage is ongoing. The absence of significant differences between the lineages thus far leaves open the question of what is driving the frequencies of the two lineages in natural populations.

Primary Faculty Mentor Name

Sara Helms Cahan

Status

Undergraduate

Student College

College of Arts and Sciences

Program/Major

Biological Science

Primary Research Category

Biological Sciences

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How do populations of two hybridizing harvester ants coexist with one another?

Interbreeding with another species is usually a costly mistake, as interspecific hybrids are often sterile. For social insects such as ants, however, hybrids can be advantageous if they develop into the worker caste, which do not directly reproduce. In the harvester ant genus Pogonomyrmex, queens of two different genetic lineages, J1 and J2, mate with and store sperm from males of both their own and the other lineage; hybrid females develop as sterile workers, while pure-lineage females develop as queens. In places where they interact, the proportion of each lineage is expected to equalize at 50:50 as selection acts against lineages that become too common, yet in the field the frequency of J1:J2 most often stabilizes around 40:60, suggesting that J2 colonies have an advantage. In this study, I tested four ecological advantage hypotheses to explain J2 dominance: enhanced foraging activity, colony size, territory size, and selective mating behavior. Worker activity and nest diameters were compared between 10 J1 colonies and 37 J2 colonies that were spatially mapped at a field site in Apache, Arizona. To assess mating behavior, 15 worker and 15 reproductive queen samples were collected from each colony and genotyped at 12 microsatellite markers, which were used to estimate the number of same and alternate-lineage males the queens had mated with. No significant differences were found for either worker activity or nest diameter between J1 and J2 colonies. The two lineages were randomly distributed across the site, with no difference in nearest neighbor distance between colony types. Research determining worker size and number of male mates of either lineage is ongoing. The absence of significant differences between the lineages thus far leaves open the question of what is driving the frequencies of the two lineages in natural populations.