Abstract
Sex chromosomal dose difference between sexes is often normalized by a gene regulatory mechanism called dosage compensation (DC). Studies indicate that DC mechanisms are generally effective in XY rather than ZW systems. However, DC studies in lepidopterans (ZW system) gave bewildering results. In Manduca sexta, DC was complete and in Plodia interpunctella, it was incomplete. In Heliconius species, dosage was found to be partly incomplete. In domesticated silkmoth Bombyx mori, DC studies have yielded contradictory results thus far, showing incomplete DC based on microarray data and a possible existence of DC based on recent reanalysis of same data. In this study, analysis of B. mori sexed embryos (78, 96 and 120 h) and larval heads using RNA sequencing suggest an onset of DC at 120 h. The average Z-linked expression is substantially less than autosomes, and the male-biased Z-linked expression observed at initial stages (78 and 96 h) gets almost compensated at 120 h embryonic stage and perfectly compensated in heads. Based on these findings, we suggest a complete but an unconventional type of DC, which may be achieved by reduced Z-linked expression in males (ZZ). To our knowledge, this is the first next-generation sequencing report showing DC in B. mori, clarifying the previous contradictions.
Highlights
Sex chromosomes are believed to have evolved from autosomes through intermediate proto-sex chromosomes
RNA sequencing (RNA-seq) of three sexed embryonic stages (78, 96 and 120 h), fifth-instar larval heads and BmN cells of B. mori resulted in 718 M reads (359 M paired) of 100 bp length from nine samples ranging from 66 to 126 M paired-end reads
We have found that the expression level of masc is relatively higher (6.45-fold) in male embryos compared with female embryos at 96 h of development, a key gene, having roles in both dosage compensation (DC) and in sex determination
Summary
Sex chromosomes are believed to have evolved from autosomes through intermediate proto-sex chromosomes. In the process of acquiring sex-determination function [2], the accumulation of sexually antagonistic mutations and repeat elements by Y (W) would have mostly contributed for the loss of homology with X/Z Such loss of homology is believed to be the driving force for Y (W) recombination isolation [3] and its degeneration via gene loss [4], leaving exclusive sex-determination function [5]. The evolution of Y for establishing two distinct sexes, males (XY) and females (XX), had resulted in ‘X chromosome aneuploidy’ [6] This aneuploidy creates the dose difference in X-linked genes, which could be deleterious if not compensated at the level of expression. Organisms have adopted a versatile gene regulatory mechanism called dosage compensation (DC), which is limited mostly to homogametic sex (X/Z) chromosomes in most species [7]
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