Polak Lab

One focus of the lab is the function and evolution of "Darwinian" secondary sexual traits—the extravagant and rapidly evolving features of animals used for mate attraction and contest competition. We test prominent theoretical models for the evolution of this spectacular class of traits, including how expression of such display traits may reveal disease resistance to prospective mates.

Projects in this area have studied the male sex "combs" in different species, and other courtship traits are also under ongoing study. Students working on the topics develop skills in genetics, and physiological assays of body reserves and reproductive traits (e.g., sperm viability and number). These advances reveal how secondary sexual traits might signal male genetic quality, fertility and other measures of post-copulatory fitness.

Together with the above efforts, the lab is pursuing ongoing projects looking at the role of fluctuating asymmetry (FA) in signaling phenotypic quality in sexual selection. FA reveals developmental instability, the ability of an organism to buffer genetic and environmental stress during development. Our field work takes us to various parts of the South Pacific, Australia and SE Asia.

How parasites fuel host sexual selection and affect secondary sexual trait expression are long-standing interests. What are the selective forces driving the evolution of mate choice for parasite-free males? How does an organism's overall health status and physiological condition interact to affect susceptibility, disease development and sexual competency? These are pertinent questions with applications to a variety of basic and applied disciplines.

The lab currently holds a collaborative NSF grant examining genetic, physiological and environmental bases of costs of parasite resistance in Drosophila. Costs (trade-offs) are central to discussions of resistance evolution because of their expected importance for the maintenance of genetic polymorphisms underlying parasitic disease and spread. A main goal of this collaborative project with co-PI Josh Benoit is to compare and contrast parasite resistance mechanism and their underlying genetic and metabolomic bases between two distinct host-parasite systems -- one in Asia and the other in North America. The research is contributing to our general understanding of the genetic bases and environmental modulation of costs of resistance in insects.

We are especially keen on the role of sexually transmitted parasites (STPs) in host evolution. In such fascinating systems, direct selection acts on female choice to avoid acquiring transmissible parasites from males. We've recently discovered a brand new system in rainforests of SE Asia to explore questions into the evolutionary consequences of sexually transmissible agents of disease (yes, you bet, in fruit flies!).

Our recent genomic profiling studies reveal that infection by ectoparasites triggers up-regulation of a cascade of host defensive genes at the expense of factors governing sperm and/or seminal fluid quality. This suggests functional links between parasitism and male fertilizing capacity. To what extent are such "cryptic" yet potent reproductive costs of immune system activation modulated by host diet and nutritional history? Do flies self-medicate to enhance their fertilizing capacity in a shifting costs landscape? These are exciting questions within the flourishing field of ecological immunology.

Fly-parasite systems are ideal for students to use in high-impact projects on a variety of topics in evolution and sexual selection.