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The causes and costs of intersexuality in two freshwater populations of the amphipod, Gammarus minus found in Montgomery County, Virginia USA

Student thesis: Doctoral Thesis

Intersexuality, the abnormal condition whereby gonochoristic animals display characteristics of both males and females, is common but not overly prevalent amongst the animal kingdom. However, females of the amphipod Gammarus minus inhabiting two freshwater springs in Virginia (USA) exhibit unusually high (60 – 100%) frequencies of intersexuality. The overall aim of my study is to explain these unusually high frequencies by identifying potential cause(s) and reproductive costs of intersexuality in these two study populations. Sex determination mechanisms in Crustacea are complex but, what is known is that sex determination can be influenced by any or all of the following mechanisms, parasitic infections, genetics (i.e. polyfactorial systems), and environmental factors including anthropogenic disruption, all of which may occur singularly or simultaneously within a species.
Currently known causes of intersexuality are considered appropriate starting points to evaluate these two populations. The effect of endocrine disruption is evident within reproductive development of the following: fecundity, fertility, growth, maturation and development, pre-copular behavior and atypical phenotypes. I employed field methods to evaluate the effect of intersexuality on population dynamics and laboratory methods to investigate the prevalence of parasites and costs of the intersex condition. Using the data from my field and laboratory investigation, the findings suggest that sex ratios may be driven by photoperiod. In both populations, the expectation that the cost associated with female intersexuality effects reproductive fitness is accurate, when compared with normal female populations.
Intersex specimen’s body lengths were consistently larger than their respective sexual phenotypes, and intersex females produced 29% fewer eggs than normal females. Intersex females matured at a greater size than normal females indicating a delay to maturity, which was verified by growth and development patterns between intersex and normal females. Normal female patterns for growth, maturation, development and fecundity were continuous with no obvious pattern, while intersex females from both populations are similar in a pattern that peaks early autumn to winter. Contrary to my expectation, pre-copular behavioral experiments regarding the costs of intersexuality of normal versus intersex females revealed that males had no preference for normal females over intersex females. However, when males that are originally paired with intersex females are separated, it takes twice as long to rejoin when compared with the normal females and males.
The findings of this study offer insights into sex determination and the possible cause of intersexuality of these two populations, as well as reproductive costs. The choice of observations regarding the effects of intersexuality (fecundity, fertility, growth, maturation and pre-copular behavioral experiments) was appropriate for evaluating the effect and costs of the intersex condition when compared with normal females.Regardless, more molecular methods are needed to identify the newly revealed microsporidian parasite. While the underlying cause of the intersex condition in both populations suggests photoperiod, it is also likely that the populations are influenced by a novel feminizing parasite. The unique prevalence of intersex G. minus allows great opportunities to explore the avenues of known causes of intersexuality, as well as unknown causes.
Original languageEnglish
Awarding Institution
Supervisors/Advisors
  • Professor Alex Ford (Supervisor)
  • Douglas S. Glazier (External person) (Supervisor)
  • Roger Downer (External person) (Supervisor)
Award dateJun 2020

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