Abstract
BackgroundRecent human transcriptomic analyses revealed a very large number of testis-enriched genes, many of which are involved in spermatogenesis. This comprehensive transcriptomic data lead us to the question whether positive selection was a decisive force influencing the evolution and variability of testis-enriched genes in humans. We used two methodological approaches to detect different levels of positive selection, namely episodic positive diversifying selection (i.e., past selection) in the human lineage within primate phylogeny, potentially driven by sperm competition, and recent positive directional selection in contemporary human populations, which would indicate adaptation to different environments.ResultsIn the human lineage (after correction for multiple testing) we found that only the gene TULP2, for which no functional data are yet available, is subject to episodic positive diversifying selection. Using less stringent statistical criteria (uncorrected p-values), also the gene SPATA16, which has a pivotal role in male fertility and for which episodes of adaptive evolution have been suggested, also displays a putative signal of diversifying selection in the human branch. At the same time, we found evidence for recent positive directional selection acting on several human testis-enriched genes (MORC1, SLC9B1, ROPN1L, DMRT1, PLCZ1, RNF17, FAM71D and WBP2NL) that play important roles in human spermatogenesis and fertilization. Most of these genes are population-specifically under positive selection.ConclusionEpisodic diversifying selection, possibly driven by sperm competition, was not an important force driving the evolution of testis-enriched genes in the human lineage. Population-specific, recent positive directional selection suggests an adaptation of male reproductive genes to different environmental conditions. Positive selection acts on eQTLS and sQTLs, indicating selective effects on important gene regulatory functions. In particular, the transcriptional diversity regulated by sQTLs in testis-enriched genes may be important for spermatocytes to respond to environmental and physiological stress.
Highlights
Recent human transcriptomic analyses revealed a very large number of testis-enriched genes, many of which are involved in spermatogenesis
Positive diversifying selection of testis-enriched genes in the human lineage Previous studies found that the genes PRM1, PRM2, ESX homeobox 1 (ESX1), spermatogenesis associated 16 (SPATA16), CATSPER1, ZAN, and PKDREJ evolve rapidly in the human lineage [18, 20,21,22,23,24,25,26]
The original hypothesis that these genes in the human lineage are under positive selection was not supported by the adaptive branch-site random effects likelihood (aBSREL) analysis because the human branches had, after correction for multiple testing, test pvalues > 0.05
Summary
Recent human transcriptomic analyses revealed a very large number of testis-enriched genes, many of which are involved in spermatogenesis. The key male reproductive organ in humans is the testes. They have two main functions: the efficient production of sperm (spermatogenesis) over a male’s reproductive life span and the synthesis of hormones necessary to develop male sex characteristics. Spermatogenesis takes place in the testis within the seminiferous tubules, supported by Sertoli cells This process comprises highly complex cellular events in which the proliferation and maturation of germ cells, derived from selfrenewing stem cells, produces about 200 million sperm daily from puberty through the entire male adulthood [1]. The mature and ejaculated spermatozoa are carried to the female tract in seminal plasma, which supports key sperm functions such as interactions with the various environments of the tubular genital tract, with the oocyte and with the female immune system and potentially helps modulate sperm rejection or tolerance [2]
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