Hemimetabolous insects elucidate the origin of sexual development via alternative splicing

Abstract

25Insects are the only animals in which sexual differentiation is controlled by sex-specific RNA 26 splicing. The doublesex (dsx) transcription factor produces distinct male and female protein 27 isoforms (DsxM and DsxF) under the control of the RNA splicing factor transformer (tra). tra 28 itself is also alternatively spliced so that a functional Tra protein is only present in females; thus, 29 DsxM is produced by default, while DsxF expression requires Tra. The sex-specific Dsx 30 isoforms are essential for both male and female sexual differentiation. This pathway is 31 profoundly different from the molecular mechanisms that control sex-specific development in 32 other animal groups. In animals as different as vertebrates, nematodes, and crustaceans, sexual 33 differentiation involves male-specific transcription of dsx-related transcription factors that are 34 not alternatively spliced and play no role in female sexual development. To understand how the 35 unique splicing-based mode of sexual differentiation found in insects evolved from a more 36 ancestral transcription-based mechanism, we examined dsx and tra expression in three basal, 37 hemimetabolous insect orders. We find that functional Tra protein is limited to females in the 38 kissing bug Rhodnius prolixus (Hemiptera), but is present in both sexes in the louse Pediculus 39 humanus (Phthiraptera) and the cockroach Blattella germanica (Blattodea). Although 40 alternatively spliced dsx isoforms are seen in all these insects, they are sex-specific in the 41 cockroach and the kissing bug but not in the louse. In B. germanica, RNAi experiments show 42 that dsx is necessary for male, but not female, sexual differentiation, while tra controls female 43 development via a dsx-independent pathway. Our results suggest that the distinctive insect 44 mechanism based on the tra-dsx splicing cascade evolved in a gradual, mosaic process: sex-45 specific splicing of dsx predates its role in female sexual differentiation, while the role of tra in 46 regulating dsx splicing and in sexual development more generally predates sex-specific 47 52 53 Sex determination and sexual differentiation evolve on drastically different time scales. Sex 54 determination (that is, the primary signal directing the embryo to develop as male or female) can 55 be either environmental or genetic. Genetic sex determination can in turn be either male-or 56 female-heterogametic (with or without heteromorphic sex chromosomes), mono-or polygenic, or 57 haplo-diploid, among other mechanisms (1-7). Primary sex determination is among the most 58 rapidly evolving developmental processes. In African clawed frogs, medaka, salmon, and other 59 animals, there are many examples of recently evolved sex-determining genes, so that the primary 60 sex determination genes can differ between closely related species (8-13). The signals initiating 61 male or female development can vary even within species, as seen in cichlids, zebrafish, house 62 flies, and Ranid frogs (5,(14)(15)(16)(17)(18). 63In contrast, sex...

Evolutionary origin of sex differentiation system in insects

“…Based on a recent phylogenetic hypothesis (Beutel et al, 2017;Misof et al, 2014), the transition from the single-to dual-functionality is estimated to have occurred in the common ancestor of Aparaglossata (holometabolan insects excluding Hymenoptera) (Figure 6A). At the phenotypic level, our findings ensure the evolutionary model by Wexler et al (2019) that dsx initially acquired sexspecific isoforms and later became essential for female differentiation.…”

“…There have been two known types of outputs of the insect sex differentiation system: one that can regulate feminization via both transcription-regulation and phenotypes, and one that cannot. The former is found in Diptera, Lepidoptera, and Coleoptera (e.g., Luo and Baker, 2015;Suzuki et al, 2003;Shukla and Palli, 2012), the latter in Dictyoptera and Hemimetabola (Wexler et al, 2019;Zhuo et al, 2018). In some species, such as the dung beetle Onthophagus taurus, dsx contributes only to male trait formation in some tissues (Ledón-Rettig et al, 2017).…”

“…In pterygote insects (e.g., Burtis and Baker, 1989;Shukla and Palli, 2012;Mine et al, 2017;Ohbayashi et al, 2001;Takahashi et al, 2019;Wexler et al, 2019;Zhuo et al, 2018), dsx has sex-specific isoforms, except for a termite Reticulitermes speratus (Miyazaki et al, 2021). In contrast, dsx is controlled via a male-specific transcription in chelicerates and crustaceans (Kato et al, 2011;Li et al, 2018;Panara et al, 2019;Pomerantz et al, 2015).…”

“…Thus, there is a difference in outputs in the sex differentiation systems in insects: promoting only male differentiation (single-functionality) or both male and female differentiation (dual-functionality). It is suggested that the output of the insect sex differentiation systems transited from the single-functionality to the dualfunctionality (Wexler et al, 2019). However, it remains largely unclear how the difference in the output evolved independently of changes in gene repertoires and what drove such a transition (Hopkins and Kopp, 2021).…”

Tissue-specific developmental regulation and isoform usage underlie the role ofdoublesexin sex differentiation and mimicry inPapilioswallowtails

“…The added complexity of having multiple female isoforms might assist dsx in partitioning some of its functions and enabling tissue-specific regulation. Besides Lepidoptera, Coleoptera and Blattodea have two female-specific exons of dsx [51,58]. Therefore, these insect groups may have multiple female isoforms, with the inclusion of one or both female exons.…”