Fig 1.
Intercellular bridges connect developing male germ cells.
These bridges are established after commitment to differentiation and ensure cytoplasmic continuity between the progeny of all subsequent mitotic and meiotic divisions. Sperm cell individualisation only occurs immediately prior to the release of the fully differentiated sperm from the gametogenic tissue (spermiation). The intercellular bridges are large enough to accommodate the flow of mRNAs, proteins, and even organelles between cells. Note that the degree of sharing of any given product is determined by a source–sink process, potentially resulting in increased levels near the site of production (as illustrated by the two cytoplasm hues in postmeiotic cells, matching the sex chromosome they contain). mRNA, messenger RNA.
Fig 2.
Tlr7/8 transcripts are detected at extremely low levels during spermatogenesis.
(A) mRNA levels in an RNAseq data set analysing different stages of pre- to postmeiotic sperm cell development [28], measured in FPKM. Premeiotic corresponds to purified spermatogonia, meiotic corresponds to spermatocytes, and postmeiotic corresponds to round spermatids. Transcript levels were recalculated from raw data and are plotted as mean log2(FPKM+1) ± SD. Tlr7/8 are depicted by coloured circles; previously reported genes that escape full sharing (Spam1, Smok2a/2b, and Smpd1) are indicated by triangles; and squares denote a well-shared postmeiotic control gene (Prm1). (B) mRNA levels in an RNAseq data set analysing two different stages of postmeiotic sperm cell development [29]. Early corresponds to round spermatids, and late corresponds to elongating spermatids. Transcript levels were recalculated from raw data and plotted in FPKM as in panel A. Inserts (a’) and (b’) provide additional detail on the low expression levels of TLR7/8. FPKM, fragments per kilobase of transcript per million reads; mRNA, messenger RNA; Prm1, protamine 1; RNAseq, RNA sequencing; Smok2a/2b, Sperm motility kinase 2a/2b; Smpd1, Sphingomyelin phosphodiesterase 1; Spam1, Sperm adhesion molecule 1; Tlr, Toll-like receptor.
Fig 3.
Percent amino acid identity between TLR orthologs of increasing evolutionary divergence.
Autosomal TLR2, TLR4 (grey), and X-linked TLR7 and 8 (red and pink) are shown. Each data point corresponds to the percent pairwise identity between mouse (Mus musculus) and the following species: Mus caroli (7.4 MY), Rattus norvegicus (20.9 MY), Meriones unguiculatus (28.6 MY), Microtus ochrogaster or Mesocricetus auratus (33 MY), Spalax galili (45 MY), Jaculus jaculus (54.8 MY), Castor canadensis (70 MY), Ictidomys tridecemlineatus (71 MY), and Cavia porcellus (73 MY). MY, million years; TLR, Toll-like receptor.