Rotation of sex combs in Drosophila melanogaster requires precise and coordinated spatio-temporal dynamics from forces generated by epithelial cells

Homeotic transformations suggest mechanisms for rapid evolution diversification in Drosophila sex combs

Evolutionary innovations refer to the emergence of new traits, functions, or behaviors in organisms and lineages over time. Although research has demonstrated that such innovations can arise gradually or through small steps (Chouard 2010), the mechanisms by which rapid morphological diversification takes place remain poorly understood (Bailey et al. 2019). To explore this question, we used the evolution of sex combs, as a system (Ho et al. 2018). We used this male-specific row of leg bristles, comprising sex combs as a system, because it displays spectacular morphological diversification in a short time (Kopp 2011). Homeotic mutations in the fruit fly, Drosophila melanogaster, are those which create modifications in one part of a fly to resemble another region. Here we describe effects of some of these mutations which transform the D. melanogaster fly sex comb morphology to closely resemble sex comb morphology in other species. These findings support previous research indicating that minor alterations to regulatory elements can play a significant role in explaining morphological evolution (Atallah et al. 2004). Thus, our results suggest that rapid diversification may not require starting from scratch, but rather may require minor modifications to the sex comb ground plan, which may account for its rapid morphological evolution (Lee et al. 2011).

Genes and genomes and unnecessary complexity in precision medicine

The sequencing of the human genome heralded the new age of 'genetic medicine' and raised the hope of precision medicine facilitating prolonged and healthy lives. Recent studies have dampened this expectation, as the relationships among mutations (termed 'risk factors'), biological processes, and diseases have emerged to be more complex than initially anticipated. In this review, we elaborate upon the nature of the relationship between genotype and phenotype, between chance-laden molecular complexity and the evolution of complex traits, and the relevance of this relationship to precision medicine. Molecular contingency, i.e., chance-driven molecular changes, in conjunction with the blind nature of evolutionary processes, creates genetic redundancy or multiple molecular pathways to the same phenotype; as time goes on, these pathways become more complex, interconnected, and hierarchically integrated. Based on the proposition that gene-gene interactions provide the major source of variation for evolutionary change, we present a theory of molecular complexity and posit that it consists of two parts, necessary and unnecessary complexity, both of which are inseparable and increase over time. We argue that, unlike necessary complexity, comprising all aspects of the organism's genetic program, unnecessary complexity is evolutionary baggage: the result of molecular constraints, historical circumstances, and the blind nature of evolutionary forces. In the short term, unnecessary complexity can give rise to similar risk factors with different genetic backgrounds; in the long term, genes become functionally interconnected and integrated, directly or indirectly, affecting multiple traits simultaneously. We reason that in addition to personal genomics and precision medicine, unnecessary complexity has consequences in evolutionary biology.

Modular tissue-specific regulation of doublesex underpins sexually dimorphic development in Drosophila

The ability of a single genome to produce distinct and often dramatically different male and female forms is one of the wonders of animal development. In Drosophila melanogaster, most sexually dimorphic traits are controlled by sex-specific isoforms of the doublesex (dsx) transcription factor, and dsx expression is mostly limited to cells that give rise to sexually dimorphic traits. However, it is unknown how this mosaic of sexually dimorphic and monomorphic organs arises. Here, we characterize the cis-regulatory sequences that control dsx expression in the foreleg, which contains multiple types of sex-specific sensory organs. We find that separate modular enhancers are responsible for dsx expression in each sexually dimorphic organ. Expression of dsx in the sex comb is co-regulated by two enhancers with distinct spatial and temporal specificities that are separated by a genitalia-specific enhancer. The sex comb-specific enhancer from D. willistoni, a species that primitively lacks sex combs, is not active in the foreleg. Thus, the mosaic of sexually dimorphic and monomorphic organs depends on modular regulation of dsx transcription by dedicated cell type-specific enhancers.