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1. Support for faster and more adaptive Z chromosome evolution in two divergent lepidopteran lineages

   Mongue, Andrew J. ; Hansen, Megan E. ; Walters, James R. ( July 2021 , Evolution)

   null (Ed.)

   The rate of divergence for Z or X chromosomes is usually observed to be greater than autosomes, but the proposed evolutionary causes for this pattern vary, as do empirical results from diverse taxa. Even among moths and butterflies (Lepidoptera), which generally share a single-origin Z chromosome, the handful of available studies give mixed support for faster or more adaptive evolution of the Z chromosome, depending on the species assayed. Here, we examine the molecular evolution of Z chromosomes in two additional lepidopteran species: the Carolina sphinx moth and the monarch butterfly, the latter of which possesses a recent chromosomal fusion yielding a segment of newly Z-linked DNA. We find evidence for both faster and more adaptive Z chromosome evolution in both species, though this effect is strongest in the neo-Z portion of the monarch sex chromosome. The neo-Z is less male-biased than expected of a Z chromosome, and unbiased and female-biased genes drive the signal for adaptive evolution here. Together these results suggest that male-biased gene accumulation and haploid selection have opposing effects on long-term rates of adaptation and may help explain the discrepancies in previous findings as well as the repeated evolution of neo-sex chromosomes in Lepidoptera. 

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2. Sitticine jumping spiders: phylogeny, classification, and chromosomes (Araneae, Salticidae, Sitticini)

   https://doi.org/10.3897/zookeys.925.39691  

   Maddison, Wayne P. ; Maddison, David R. ; Derkarabetian, Shahan ; Hedin, Marshal ( April 2020 , ZooKeys)

   The systematics of sitticine jumping spiders is reviewed, with a focus on the Palearctic and Nearctic regions, in order to revise their generic classification, clarify the species of one region (Canada), and study their chromosomes. A genome-wide molecular phylogeny of 23 sitticine species, using more than 700 loci from the arachnid Ultra-Conserved Element (UCE) probeset, confirms the Neotropical origins of sitticines, whose basal divergence separates the new subtribe Aillutticina (a group of five Neotropical genera) from the subtribe Sitticina (five genera of Eurasia and the Americas). The phylogeny shows that most Eurasian sitticines form a relatively recent and rapid radiation, which we unite into the genus Attulus Simon, 1868, consisting of the subgenera Sitticus Simon, 1901 (seven described species), Attulus (41 described species), and Sittilong Prószyński, 2017 (one species). Five species of Attulus occur natively in North America, presumably through dispersals back from the Eurasian radiation, but an additional three species were more recently introduced from Eurasia. Attus palustris Peckham & Peckham, 1883 is considered to be a full synonym of Euophrys floricola C. L. Koch, 1837 (not a distinct subspecies). Attus sylvestris Emerton, 1891 is removed from synonymy and recognized as a senior synonym of Sitticus magnus Chamberlin & Ivie, 1944. Thus, the five native Attulus in North America are Attulus floricola , A. sylvestris , A. cutleri , A. striatus , and A. finschi . The other sitticines of Canada and the U.S.A. are placed in separate genera, all of which arose from a Neotropical radiation including Jollas Simon, 1901 and Tomis F.O.Pickard-Cambridge, 1901: (1) Attinella Banks, 1905 ( A. dorsata , A. concolor , A. juniperi ), (2) Tomis ( T. welchi ), and (3) Sittisax Prószyński, 2017 ( S. ranieri ). All Neotropical and Caribbean “ Sitticus ” are transferred to either Jollas (12 species total) or Tomis (14 species). Attinella (three species) and Tomis are both removed from synonymy with Sitticus ; the synonymy of Sitticus cabellensis Prószyński, 1971 with Pseudattulus kratochvili Caporiacco, 1947 is restored; Pseudattulus Caporiacco, 1947 is synonymized with Tomis . Six generic names are newly synonymized with Attulus and one with Attinella . Two Neotropical species are described as new, Jollas cupreus sp. nov. and Tomis manabita sp. nov. Forty-six new combinations are established and three are restored. Three species synonymies are restored, one is new, and two are rejected. Across this diversity of species is a striking diversification of chromosome complements, with X-autosome fusions occurring at least four times to produce neo-Y sex chromosome systems (X 1 X 2 Y and X 1 X 2 X 3 Y), some of which ( Sittisax ranieri and S. saxicola ) are sufficiently derived as to no longer preserve the simple traces of ancestral X material. The correlated distribution of neo-Y and a base autosome number of 28 suggests that neo-Y origins occurred preferentially in lineages with the presence of an extra pair of autosomes. 

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3. Retrogene Duplication and Expression Patterns Shaped by the Evolution of Sex Chromosomes in Malaria Mosquitoes

   https://doi.org/10.3390/genes13060968  

   Miller, Duncan ; Chen, Jianhai ; Liang, Jiangtao ; Betrán, Esther ; Long, Manyuan ; Sharakhov, Igor V. ( June 2022 , Genes)

   Genes that originate during evolution are an important source of novel biological functions. Retrogenes are functional copies of genes produced by retroduplication and as such are located in different genomic positions. To investigate retroposition patterns and retrogene expression, we computationally identified interchromosomal retroduplication events in nine portions of the phylogenetic history of malaria mosquitoes, making use of species that do or do not have classical sex chromosomes to test the roles of sex-linkage. We found 40 interchromosomal events and a significant excess of retroduplications from the X chromosome to autosomes among a set of young retrogenes. These young retroposition events occurred within the last 100 million years in lineages where all species possessed differentiated sex chromosomes. An analysis of available microarray and RNA-seq expression data for Anopheles gambiae showed that many of the young retrogenes evolved male-biased expression in the reproductive organs. Young autosomal retrogenes with increased meiotic or postmeiotic expression in the testes tend to be male biased. In contrast, older retrogenes, i.e., in lineages with undifferentiated sex chromosomes, do not show this particular chromosomal bias and are enriched for female-biased expression in reproductive organs. Our reverse-transcription PCR data indicates that most of the youngest retrogenes, which originated within the last 47.6 million years in the subgenus Cellia, evolved non-uniform expression patterns across body parts in the males and females of An. coluzzii. Finally, gene annotation revealed that mitochondrial function is a prominent feature of the young autosomal retrogenes. We conclude that mRNA-mediated gene duplication has produced a set of genes that contribute to mosquito reproductive functions and that different biases are revealed after the sex chromosomes evolve. Overall, these results suggest potential roles for the evolution of meiotic sex chromosome inactivation in males and of sexually antagonistic conflict related to mitochondrial energy function as the main selective pressures for X-to-autosome gene reduplication and testis-biased expression in these mosquito lineages. 

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4. The Diversity and Evolution of Sex Chromosomes in Frogs

   https://doi.org/10.3390/genes12040483  

   Ma, Wen-Juan ; Veltsos, Paris ( April 2021 , Genes)

   Frogs are ideal organisms for studying sex chromosome evolution because of their diversity in sex chromosome differentiation and sex-determination systems. We review 222 anuran frogs, spanning ~220 Myr of divergence, with characterized sex chromosomes, and discuss their evolution, phylogenetic distribution and transitions between homomorphic and heteromorphic states, as well as between sex-determination systems. Most (~75%) anurans have homomorphic sex chromosomes, with XY systems being three times more common than ZW systems. Most remaining anurans (~25%) have heteromorphic sex chromosomes, with XY and ZW systems almost equally represented. There are Y-autosome fusions in 11 species, and no W-/Z-/X-autosome fusions are known. The phylogeny represents at least 19 transitions between sex-determination systems and at least 16 cases of independent evolution of heteromorphic sex chromosomes from homomorphy, the likely ancestral state. Five lineages mostly have heteromorphic sex chromosomes, which might have evolved due to demographic and sexual selection attributes of those lineages. Males do not recombine over most of their genome, regardless of which is the heterogametic sex. Nevertheless, telomere-restricted recombination between ZW chromosomes has evolved at least once. More comparative genomic studies are needed to understand the evolutionary trajectories of sex chromosomes among frog lineages, especially in the ZW systems. 

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5. Did the creeping vole sex chromosomes evolve through a cascade of adaptive responses to a selfish x chromosome?

   https://doi.org/10.1002/bies.202100164  

   Roy, Scott William ( November 2023 , BioEssays)

   The creeping voleMicrotus oregoniexhibits remarkably transformed sex chromosome biology, with complete chromosome drive/drag, X‐Y fusions, sex reversed X complements, biased X inactivation, and X chromosome degradation. Beginning with a selfish X chromosome, I propose a series of adaptations leading to this system, each compensating for deleterious consequences of the preceding adaptation: (1) YY embryonic inviability favored evolution of a selfish feminizing X chromosome; (2) the consequent Y chromosome transmission disadvantage favored X‐Y fusion (“X^P”); (3) Xist‐based silencing of Y‐derived X^Pgenes favored a second X‐Y fusion (“X^M”); (4) X chromosome dosage‐related costs in X^PX^Mmales favored the evolution of X^Mloss during spermatogenesis; (5) X chromosomal dosage‐related costs in X^M0 females favored the evolution of X^Mdrive during oogenesis; and (6) degradation of the non‐recombining X^Pfavored the evolution of biased X chromosome inactivation. I discuss recurrent rodent sex chromosome transformation, and selfish genes as a constructive force in evolution.

    

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