Genome Variation Within the Social Environment

Understanding how genome variation within the social environment influences indirect genetic effects.

Indirect genetic effects (IGE) are genetic effects of an individual on traits expressed in conspecifics. Because conspecifics are an important part of the (social) environment in many organisms, IGE are expected to be common. Accumulating examples include effects on morphological, developmental, and behavioral traits in domestic species, natural genotypes, and in free-living animals and plant. IGE can have profound effects on both the evolvability of traits and on response to selection (e.g., reversing the direction of response predicted based on direct genetic variance).

Most empirical studies of IGE have focused on phenotypic effects of genetic variation in the social environment; little is known about the underlying mechanisms. If traits of an individual are affected by IGE, it must be because the patterns of gene expression are affected, since we know gene sequences are not changing. Epigenetic modifications are most likely the mechanism by which individuals can change the expression of their genes in response to others in their social environment. Some of the most common epigenetic changes are histone modifications and DNA methylation patterns in both plants and vertebrates. These modifications within the epigenome can be transient or become fixed.

Studies of identical human twins have shown that the epigenome is almost identical when young; as twins ages the epigenomes diverge, becoming more distinct from one another. This suggests that even those individuals with the same genome express their genes differently over time as a result of their interactions with their environment. Few vertebrates naturally reproduce clonally, but those that do provide an outstanding opportunity to investigate epigenetic regulation of IGE. Amazon mollies (Poecilia formosa) are thought to have arisen from an ancient hybridization event between P. latipinna and P. mexicana. Amazons reproduce gynogenetically; although they require the sperm of closely related males to initiate embryogenesis, rarely does any male genetic material become incorporated into the genome of the offspring. The number of clonal lineages occurring within the same population can vary from one to greater than a dozen; therefore, the degree of competition and the frequency with which females encounter females of different lineages can vary greatly. Finally, the genome for the Amazons was recently published, making whole epigenomic comparative studies among different clonal lineages possible. These features make Amazons an excellent system to study questions pertaining to IGE and epigenetics.

Despite clonal reproduction, Amazons show substantial variation in mate preferences, aggression, kin recognition, shoaling preferences, foraging abilities, etc. both among and within clones. This extensive phenotypic variation is surprising in a species with a single hybrid origin and limited means to acquire genetic variation. The mechanism(s) that regulate this variation within and among clonal lineages of Amazons have yet to be identified. In previous research, indicate that individual behavior depends in part on the genetic composition of the social environment. That is, IGE provide a potential solution to the paradox of high phenotypic variation in a clonally-reproducing animals. Here I manipulate the genetic composition of the social environment and measures the resulting phenotypic and epigenetic variation. This experiment will associate IGE with underlying mechanisms of gene regulation, which, to my knowledge, has not previously been attempted. I am conducting the experiment in mesocosms that mimic biotic and abiotic features of the natural environment, but that allow replicated manipulation of the social environment, and detailed measurement of behavior and other phenotypes. Manipulations of the genetic composition of the social environment will allow me to assess both the behavioral and epigenetic consequences of IGE.

Some of the phenotypic behaviors we are measuring include: Exploratory / stress behaviors, partner perferences, shoaling behaviors, and foraging behaviors (see videos below).

This research was supported via a Postdoctoral fellowship from the Provost Postdoctoral Fellow Program and completed in collaboration with Dr. Kimberly Hughes at Florida State University.