The evolution of nomadic swarm raiders: Determining the genetic component of army ant castes

2013 REU Intern Andrew Burchill


Junior Biology (Ecology and Evolution) major at The University of Chicago

REU Mentors: Dr. Corrie Moreau (Assistant Curator, Zoology/Insects) and Max Winston (Ph.D. graduate student, Zoology/Insects)

Symposium Presentation Title: Analyzing the Genetic Component in Caste Determination of Neotropical Army Ants

Symposium Presentation Abstract: Many organisms exhibit instances of polyphenism, in which a single genotype can result in various, discrete phenotypes, depending on environmental cues. The Neotropical army ant species Eciton burchellii provides an excellent study system for polyphenism, because as a eusocial insect, they have a large number of morphological castes present within one colony. The sterile, non-mating workers can be divided into four castes that exhibit functional specialization. Although it is believed that caste determination in army ants is primarily accomplished through different doses of juvenile hormone, recent studies suggest there may be a genetic component as well. Queens are highly polyandrous, and there is evidence that some paternal lineages may have higher propensities for developing into certain castes. In order to address this issue, 240 individuals from 10 colonies in South America were sampled. Back leg lengths were measured and used as a proxy for individual body size and caste. DNA was also extracted and three microsatellite loci were used to assign patrilines in the colonies. Interpatriline variation could then be statistically assessed. Approximately 106 patrilines were detected, a larger number than what other studies have estimated, implying that queens may be even more polyandrous than previously believed. Initial analyses suggest that there is no genotypic bias on caste phenotype, although increased sampling is needed for a more robust analysis. In future research, geometric morphometrics could be applied to further characterize morphological variation and caste division in Eciton burchellii. Workers born from a single cohort should also be sampled to control for time-related effects and partriline shifting.

Original Project Description: The army ant Eciton burchellii feeds in nomadic swarms and are keystone social predators of Neotropical rainforests. Workers leave the nest in groups of up to several hundred thousand individuals to dismember live prey, exerting strong top-down effects on community structure in the leaf litter. Undoubtedly, a large part of their ecological success can be attributed to their complex caste system, featuring morphologically distinct soldiers that play a critical role in colony defense. Although it is generally held that the diet and rearing environment of developing larvae determines the physical caste of adult individuals, highly polyandrous systems have shown that caste determination might have a significant genetic component. We will use comparative morphometrics to investigate the morphological variation in individuals of different castes, and link this variation to genetic microsatellite data to understand genetic contribution to caste determination in 16 E. burchellii colonies. Considering an average E. burchellii army ant queen mates with 13 males, one of the central questions of this research will be whether certain patrilines have a greater propensity to develop into a particular caste. Furthermore, whether the morphological variation observed in certain patrilines is fundamentally different than other patrilines.

Research methods and techniques: Interns will receive training in imaging, specimen mounting, digital measurement software, geometric morphometric analysis, and multivariate statistics. Moreover, interns will learn how to analyze microsatellite data, and understand the mechanisms behind this molecular method. Depending on spring work flow, there is a possibility that the intern will also receive training in DNA extraction, PCR, and fragment analysis in the Museum’s core genetics facility, the Pritzker Laboratory and the DNA Discovery Center.