Sexual size dimorphism predicts rates of sequence evolution of SPerm Adhesion Molecule 1 (SPAM1, also PH-20) in monkeys, but not in hominoids (apes including humans)

Based on a dataset comprising coding DNA sequences of 23 anthropoid primates, we herein investigate if rates of sequence evolution of SPerm Adhesion Molecule 1 (SPAM1, also PH-20), which participates in sperm–egg interaction, is lower in more sexually dimorphic species. For comparison, we analyze sequence evolution of apolipoprotein A-IV (APOA4) and apolipoprotein A-V (APOA5), which should evolve under less or even no sexual selection given their expression in blood, digestive tract, liver, and lungs. Regression analyses provides significant support for a negative dependence of SPAM1 derived branch-specific ratios of non-synonymous to synonymous substitution rates (dN/dS) on sexual size dimorphism (SSD) in a subsample comprising New World and Old World monkeys. We moreover observed a tendency for a positive correlation of substitution rates of SPAM1 with relative testes weight (RTW) and significantly lowered dN/dS estimates in uni-male and uni-male/multi-male breeding monkeys. Importantly, the pattern was not reproduced when analyzing partial APOA4 and APOA5 sequences. These findings illustrate that different levels of sperm competition, probably fueled by female cryptic choice, account for species-specific sequence evolution of SPAM1 in monkeys. Remarkably, present data do not support a correlation of species-specific sequence evolution of SPAM1 with sexual selection levels in hominoids (apes including humans). This can partly be ascribed to a relaxation of functional constraint of SPAM1 in some hominoid species. Additional factors confounding regression analyses specifically in hominoids might be higher levels of sperm competition than reflected by SSD and RTW in some species, a rather strong effect of female mate choice on paternity rates in others, and – in particular in humans – socio-cultural factors not measurable by SSD and RTW.

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► Sequence evolution of the protein Sperm Adhesion Molecule 1 (SPAM1) was analyzed at the cDNA level.
► SPAM1 derived dN/dS values depend negatively on sexual size dimorphism in a sample of New World and Old World monkeys.
► SPAM1 derived dN/dS values tentatively correlate with relative testes weight in a sample of New World and Old World monkeys.
► Uni-male and uni-male/multi-male breeding New World and Old World monkey species show significantly lowered dN/dS values.
► The pattern is not reproduced in hominoids, neither in analog approaches on APOA4/5.