Male mice with large inversions or deletions of X-chromosome palindrome arms are fertile and express their associated genes during post-meiosis

Abstract

Large (>10 kb) palindromic sequences are enriched on mammalian sex chromosomes. In mice, these palindromes harbor gene families (≥2 gene copies) expressed exclusively in post-meiotic testicular germ cells, a time when most single-copy sex-linked genes are transcriptionally repressed. This observation led to the hypothesis that palindromic structures or having ≥2 gene copies enable post-meiotic gene expression. We tested these hypotheses by using CRISPR to precisely engineer large (10’s of kb) inversions and deletions of X-chromosome palindrome arms for two regions that carry the mouse 4930567H17Rik and Mageb5 palindrome gene families. We found that 4930567H17Rik and Mageb5 gene expression is unaffected in mice carrying palindrome arm inversions and halved in mice carrying palindrome arm deletions. We assessed whether palindrome-associated genes were sensitive to reduced expression in mice carrying palindrome arm deletions. Male mice carrying palindrome arm deletions are fertile and show no defects in post-meiotic spermatogenesis. Together, these findings suggest palindromic structures on the sex chromosomes are not necessary for their associated genes to evade post-meiotic transcriptional repression and that these genes are not sensitive to reduced expression levels. Large sex chromosome palindromes may be important for other reasons, such as promoting gene conversion between palindrome arms.

Spermatogenesis and the Evolution of Mammalian Sex Chromosomes

“…This model posits a relationship between ampliconic expansions, overall expression levels, and sperm development. However, a recent study showed that deleting copies of ampliconic genes on the mouse X-chromosome did not reduce fertility, despite reducing gene expression levels by half [82]. Thus, the functional relationship between postmeiotic amplicon evolution and spermatogenesis is still unclear.…”

Large X-linked palindromes undergo arm-to-arm gene conversion across Mus lineages

“…M. musculus contains eight singleton X-palindromes with arms spanning 13-65kb in length (Kruger, et al 2018). We screened M. spretus (CHORI-35 SPRET/Ei) male mouse (XY) highdensity filters of the BAC library using hybridization probes.…”

X-linked palindromic gene families 4930567H17Rik and Mageb5 are dispensable for male mouse fertility

“…To generate mice lacking the 4930567H17Rik gene family (4930567H17Rik ∆CDS/Y ), C57BL/6J X SJL hybrid females were crossed with existing 4930567H17Rik +/Y mice [12].…”

“…The limited number of differentially expressed genes, suggests these differentially regulated genes are involved in spermatogenesis suggests Thus, 4930567H17Rik or Mageb5 influence the transcription of a limited to a small collection of genes. S3), including a normal appearance of round spermatids, the cell type in which 4930567H17Rik and Mageb5 are predominantly expressed [3,12]. In agreement with this finding, no significant differences in testis size (assayed via testis/body mass ratios) were detected (4930567H17Rik ∆CDS/Y = 0.006428±0.00086, 4930567H17Rik +/Y =0.006167±0.00067, p=0.47; Mageb5 ∆Arm∆CDS/Y =0.007342±0.00050, Mageb5 +/Y =0.007163±0.00069, p=0.54) (Fig 3B).…”