Sexual Dimorphism
All pairs of animals in the above photos are males and females of the same species, although they exhibit vastly different morphologies, physiologies and behaviors. Because males and females typically share largely the same genetic informatio, these differences in phenotypes are thought to arise from differences in gene expression. I use transcriptomic datasets (e.g., from microarray and RNAseq experiments)to understand what these differences in gene expression are.
Interactions between genetic sex, nutrition and body region
Sexual dimorphism is an important source of phenotypic variation; it is likely responsible for the greatest breadth of intraspecific variation in the natural world. Moreover, the extent of sexual dimorphism in a species can vary based on nutritional conditions and across different tissues. This extensive phenotypic variation occurs in spite of the fact that different tissues within an individual, nutritional morphs within a sex, and sexes within a species share largely the same information. Thus it is thought that this phenotypic variation arises from differences in gene expression. It is important to me, as an evolutionary biologist, to understand what these differences in gene expression are because it can tell us a lot about how evolvable such traits are, their evolutionary constraints, and even the coevolution of multiple condition-dependent and sexually dimorphic traits.
Onthophagus taurus, the bull-headed dung beetle (right), is perfect for investigating these differences in gene expression. It is sexually dimorphic, with large males producing horns and large females remaining hornless. Further, this sexual dimorphis is condition-dependent, and varies among tissues (e.g., legs are monomorphic under all conditions, while horns are not).
Modified from Kijimoto et al. 2014
Lessons learned from transcriptomic studies
From analysing these transcriptomic sets, I've learned a few basic things about the development of condition-dependent, sexually dimorphic traits,
  • Reducing trait growth in females may take as much sex-biased gene expression as does trait induction in males
  • The number of sex-biased contigs is mostly related to morpologic sexual dimorphism under high nutritional conditions, but
  • Monomorphism under low nutritional conditions may necessitate sex-biased gene expression, as well.
In this figure, the number of contigs that were significantly sex-biased (whether male- or female-biased) were plotted under both high and low conditions, and across four tissues that vary in their nutrition responsiveness; (A) abdominal epidermins, (L) legs, (T) thoracic horns, and (H) head horns.
The evolution of doublesex and its target repertoire
One of the great by-products of producing transcriptomic data sets is the abiltiy to identify candidate genes that can be functionally analyzed using tools such as RNA interference, a technique that is well-established in the Moczek lab. One such gene that has been indentified from our transcriptomic studies is doublesex, the sex determination gene. It is our hypothesis that changes in ecpression levels of doublesex and its target repertoire of genes among tissues, morphs and species is responsible for the impressive diversification of sexually dimorphic phenotypes in Onthophagine beetles.
We are testing this idea by knocking down dsx in O. taurus males and females and comparing their transcriptomes to control individuals. Further, we aim to determine whether variation in dsx expression and its interactions with downstream genes are responsible for the evolution of alternate, condition morphs (by knocking down dsx in low nutrition O. taurus males), or responsible for reversals in sexual dimorphism (by knocking down dsx in O. sagittarius, a species that exhibits condition-dependent polyphenism in females). These comparisons will allow us to answer the following questions,
  • What genes are positively or negatively regulated by dsx? Is role of dsx in generating sexually dimorphic phenotypes symmetrical or asymmetrical among the sexes?
  • Are the genes that generate hornless phenotypes in females the same that produce hornless phenotypes in males?
  • Are the genes that are positively or negatively regulated by dsx the same in O. taurus as they are in O. sagittarius?
Onthophagus sagittarius  (right) is the sister species of O. taurus, and exhibits a sex-reversal in its polyphenism: whereas females (right) possess head and thoracic horns that exhibit nutrition-dependent growth, male (left) only produce small head horns whose growth is independent of nutritional conditions.