Epigenetics as an answer to Darwin's "special difficulty".

Brian R Herb

doi:10.3389/fgene.2014.00321

Abstract

Epigenetic modifications produce distinct phenotypes from the same genome through genome-wide transcriptional control. Recently, DNA methylation in honeybees and histone modifications in ants were found to assist the formation of caste phenotypes during development and adulthood. This insight allows us to revisit one of Darwin’s greatest challenges to his natural selection theory; the derivation of multiple forms of sterile workers within eusocial species. Differential feeding of larvae creates two distinct developmental paths between queens and workers, with workers further refined by pheromone cues. Flexible epigenetic control provides a mechanism to interpret the milieu of social cues that create distinct worker sub-caste phenotypes. Recent findings suggest a distinct use for DNA methylation before and after adult emergence. Further, a comparison of genes that are differentially methylated and transcriptionally altered upon pheromone signaling suggests that epigenetics can play a key role in mediating pheromone signals to derive sub-caste phenotypes. Epigenetic modifications may provide a molecular mechanism to Darwin’s ”special difficulty” and explain the emergence of multiple sub-phenotypes among sterile individuals.

Highlights

Epigenetic modifications produce distinct phenotypes from the same genome through genome-wide transcriptional control
This insight did not fully answer the challenge, because he goes on to marvel, not at the existence of sterile workers per se, which he equates to the trait divergence between males and females, but rather how can multiple sub-phenotypes of sterile workers arise: The great difficulty lies in the working ants differing widely from both the males and the fertile females in structure, as in the shape of the thorax, and in being destitute of wings and sometimes of eyes, and in instinct
Epigenetic information can be stored in the molecular form as methylation on the cytosine base of DNA or a variety of modifications to histone tails (Kouzarides, 2007; Jones, 2012). These epigenetic modifications play a key role in tissue development where drastically different organs are derived from the same genome (Irizarry et al, 2009)

Summary

Introduction

Epigenetic modifications produce distinct phenotypes from the same genome through genome-wide transcriptional control. Studies investigating DNA methylation differences between worker subcastes in honeybees (Herb et al, 2012) and between queens and workers in ants (Bonasio et al, 2012) have found that many differentially methylated genes are involved in noncoding RNA processing, suggesting a role for non-coding RNA in caste determination.

Results

Conclusion