Variation in a pleiotropic hub gene drives morphological evolution: Insights from interspecific differences in head shape and eye size inDrosophila

Abstract

1The size and shape of an organism is tightly controlled during embryonic and 2 postembryonic development to ensure proper functionality. However, in the light of the 3 breath-taking diversity of body forms observed in nature, developmental processes must have 4 evolved to allow evolutionary changes in adult morphology. Therefore, gene regulatory 5 networks (GRNs) that orchestrate organ development are mostly constrained, but nodes and 6 edges within such networks must change to give rise to morphological divergence. Identifying 7 such tuning nodes remains a major challenge in evolutionary developmental biology. Here, we 8 combined comparative transcriptomics and chromatin accessibility data to study 9 developmental differences leading to natural variation in compound eye size and head shape 10 in the two closely related Drosophila species D. melanogaster and D. mauritiana. We show that 11 variation in expression of the GATA transcription factor Pannier (Pnr) is associated with 12 extensive remodeling of the transcriptomic landscape during head development. Since U-13 shaped (Ush), a co-factor of Pnr, is involved in the same regulatory context, we argue that 14 variation in expression of both factors may be a driver of divergence in head morphology. 15Applying functional genetics and geometric morphometrics we confirmed that manipulation of 16 pnr expression in D. melanogaster largely phenocopies D. mauritiana dorsal head shape and 17 ommatidia number. Therefore, we propose that the regulatory module composed of Pnr and 18 Ush represents a tuning node within the otherwise highly conserved GRN underlying head 19 development in Drosophila. 20 21 developmental GRNs allows gaining new insights into constrained and variable developmental 1 processes, respectively. 2One approach to reveal tuning nodes is first to identify genetic variants associated with 3 morphological diversity and then in a second step to establish regulatory interactions of genes 4 affected by those variants. However, variation in complex morphological traits is influenced by 5 many genetic loci with small effect sizes, which are spread throughout various genomic 6 locations 23-25 . For instance, mandible and craniofacial shape differences between mouse 7 strains are influenced by various loci located on most of the chromosomes [26][27][28][29] . Similarly, 8 studies in Drosophila revealed that loci on several chromosomes contribute to differences in 9 eye size and head shape [30][31][32] . These examples corroborate the complexity of the genetic 10 architecture of such traits and highlight the difficulty to determine individual causative 11 molecular changes. 12As an alternative, we propose to first identify tuning nodes within the GRN governing 13 the development of a variable morphological trait. We assume that variation in the expression 14 or function of putative tuning nodes (e.g. genes coding for transcription factors) directly affects 15 the expression of downstream target genes (i.e. "gene modules" 33 ). Candidate tuning nodes 16 can then ...

Heterochrony inorthodenticleexpression is associated with ommatidial size variation betweenDrosophilaspecies

“…Our work provides new insights into ommatidia size regulation and the evolution of eye size. Together with evidence from other studies showing that changes in the timing of ey expression contributes to differences in ommatidia number in Drosophila (Ramaekers et al, 2019), we now have a better understand the genetic and developmental mechanisms that underlie the large diversity in Drosophila eye size (Arif et al, 2013; Buchberger et al, 2021; Gaspar et al, 2020; Hilbrant et al, 2014; Keesey et al, 2019; Norry and Gomez, 2017; Posnien et al, 2012; Ramaekers et al, 2019; Reis et al, 2020). Moreover, this evidence suggests that changes in the temporal expression of upstream TFs is a widespread mechanism responsible for morphological evolution.…”

“…To investigate the regulation of otd in the developing eyes of D. simulans and D. mauritiana further, we performed ATAC-seq (Buchberger et al, 2021; Buenrostro et al, 2013; Kittelmann et al, 2018) on D. simulans and D. mauritiana eye imaginal discs at 96 and 120 h AEL. We mapped our datasets against both genomes in order to detect common, differentially accessible and species-specific regulatory regions (Fig.…”

“…For example, killer flies, Coenosia sp., have evolved wider, flattened, anterior ommatidia to maximise contrast sensitivity and acuity as an adaptation to hunting (Gonzalez-Bellido et al, 2011). A number of studies have also found extensive variation in eye size within and between closely related species of Drosophila , caused by differences in ommatidia number and/or ommatidia area (Arif et al, 2013; Buchberger et al, 2021; Gaspar et al, 2020; Hilbrant et al, 2014; Keesey et al, 2019; Norry and Gomez, 2017; Posnien et al, 2012; Ramaekers et al, 2019; Reis et al, 2020). Despite the pervasive variation in eye morphology and detailed knowledge about eye development in Drosophila (Casares and Almudi, 2016; Casares and McGregor, 2021; Kumar, 2018), little is known about the genetic and developmental bases for variation in eye size even among Drosophila species with a few exceptions (e.g.…”