BEACON Researchers at Work: Understanding how males and females grow apart

This week’s BEACON Researchers at Work blog post is by MSU graduate student Nick Testa.

Nick TestaBiology: really, it’s all about sex. In this case though, I’m talking about the actual sexes, males and females, and how they are different. Most people can spot the difference between male and female deer pretty quickly. Just look for the antlers, right? Sexually dimorphic traits, those that differ between males and females (like antlers), are incredibly widespread in nature and can have some pretty extreme variation. Most of the time these traits are sexually dimorphic because they convey some sort of benefit to one sex in particular. In this example, the antlers are not only used by male deer as weapons when competing for females, but they also act as a useful signal to females that the male deer is healthy enough to support such a big rack.

To me the coolest aspect of this is that within every species you have two, potentially very different looking creatures generated from essentially the same genome (differing only in the sex chromosome, if at all). Angler fish, for example, are extremely sexually dimorphic. Most people can identify an angler fish, but what most people don’t know is that they are probably just thinking of the female (remember Finding Nemo?). Males are up to an order of magnitude smaller than females by length (orders smaller by weight) and look like some sort of sad, bulbous minnow (Pietsch 2005). Most will eventually find, mate with, and subsequently melt into the female. Both of these creatures are the same species, but look vastly different. It’s truly amazing that the difference of a chromosome can turn a ghastly predator into a mopey parasite. I really like this example because it not only illustrates the incredible variation of sexual dimorphism of body shape, but also of body size.

Female and male anglerfish

Every multi-cellular organism has some quantifiable size and shape, which are often sexually dimorphic. An organism’s size and shape can also influence its ability to produce offspring, escape predators and even appear attractive to the other sex! These qualities make sexual dimorphism a great trait to study. Much research in this field has examined how natural selection might differ between the sexes, leading to evolutionary conflicts. That is, the ideal body size for a male and female might differ substantially, despite their largely shared genome. As for my own work, I am interested in questions involving the underlying developmental, physiological and genetic mechanisms that generate the differences in size and shape between the sexes. Understanding how these mechanisms work can allow us to further our understanding of how sexual dimorphism evolves.

Sexual size dimorphism in Drosophila melanogaster. Females (left) are larger than males (right)

Sexual size dimorphism in Drosophila melanogaster. Females (left) are larger than males (right)

The fruit fly, Drosophila melanogaster, is like most insects with regard to sexual size dimorphism; females are larger than males. In general, final body size is regulated by a combination of developmental factors, including: initial body size (usually size at hatching/birth), growth rate, growth duration and even weight loss before maturation (Testa et al. 2013). Changing any of these individually or in combination results in an alteration of adult body size. It turns out that all of these factors (except growth rate) contribute to size differences between the sexes in the fruit fly.

Size differences in fruit flies, however, are largely due to differences in their metabolic activity. Females grow faster while on food and lose more weight when they wander around looking for a spot to metamorphose. In fact, it appears that sexual size dimorphism depends on available nutrients. By rearing flies using food that varies in nutritional content we get a clear idea of how these nutrients contribute to sexual size dimorphism. We found that adult flies remained sexually size dimorphic until food quality dipped below a certain amount. Any lower and the dimorphism disappears. What’s more, we’ve also found that flies containing a mutant version of the Insulin-receptor gene not only have trouble detecting nutrients (they are effectively starved), but also develop without any sexual size dimorphism, as if they were starved (Testa et al., 2013). These results are particularly interesting for me because it suggests that sexual size dimorphism might be regulated by genetic pathways that regulate growth based on available nutrients.

Using standard genetic methods, I’ve been taking genes in candidate pathways and increasing or decreasing their functional activity to determine which ones alter sexual dimorphism. By both removing and increasing expression of these genes, I will be determining whether each one is necessary to generate sexual size dimorphism and/or sufficient to change it, respectively. Only by showing that a gene is necessary for dimorphism and sufficient to change it do we actually show that it is a causal agent.

While I am primarily interested in sexual dimorphism of whole body size, not all size determining pathways act equally on all parts of the body. Some of the genes in my candidate genetic pathways are influencing relative sex-specific changes in size, i.e. shape. For example, we know that: 1) dimorphism of overall body size in Drosophila is controlled, in part, by the sex determination pathway, 2) this pathway is also responsible for generating the morphological differences in males and females and 3) that Drosophila wings display sexual dimorphism for both size and shape. Examining how genes in these pathways (and those nearby) influence sexual dimorphism for size and shape allows me to assess the degree to which wings are under similar developmental genetic control. Using this sort of analysis I can visualize the effect each gene has on generating sex-specific size versus shape! To me, this is probably the coolest part.

Sexual shape dimorphism in wings taken from a natural population. Size effects have been removed (and shape slightly exaggerated) to demonstrate shape differences.

Sexual shape dimorphism in wings taken from a natural population. Size effects have been removed (and shape slightly exaggerated) to demonstrate shape differences.

Studying developmental mechanisms allows us to answer questions about the ‘how’ of sexual dimorphism’s evolution. How do selective forces translate into the sexual dimorphism we see in nature? How do developmental processes impact the evolution of sexual dimorphism? We know so very little about the mechanisms used to do this. In studying these mechanisms, I hope to not only learn about sexual dimorphism, but about more generalizable phenomena as well. Studying the developmental mechanisms of sexual dimorphism will inform us as to how it is, and can be, generated. More broadly, however, it can inform us about the generation of distinct phenotypes within nearly identical genomes.

References:

Pietsch, T. W. (2005). Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes). Ichthyological Research, 52(3), 207–236. doi:10.1007/s10228-005-0286-2

Testa, N. D., Ghosh, S. M., & Shingleton, A. W. (2013). Sex-Specific Weight Loss Mediates Sexual Size Dimorphism in Drosophila melanogaster. PLoS ONE, 8(3), e58936. doi:10.1371/journal.pone.0058936

For more information about Nick’s work, you can contact him at testanic at msu dot edu. 

About Danielle Whittaker

Danielle J. Whittaker, Ph.D. Managing Director of BEACON
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