Sperm competition and the evolution of sperm form and function in mammals

Abstract

Sperm competition is a powerful selective force that has influenced many reproductive traits in males and females although additional evolutionary explanations may help to understand the diversity of mammalian reproduction. Sperm morphology varies considerably in mammals with extreme examples in several rodent lineages in which a wide range of sizes and complex head morphologies have been identified. Mammalian spermatozoa also differ in function, with swimming velocity and trajectory showing much divergence. Underlying processes mediating function have received little attention so far, but differences in timing and proportion of sperm undergoing capacitation or acrosomal exocytosis may be related to variation in signalling processes. Furthermore, energy required for sperm functions (such as motion, signalling and overall maintenance of cell integrity) can be produced and consumed, following different patterns among species and this could be the result of several selective forces. A more thorough understanding of the diversity in structure and function of sperm cells, and underlying selective forces, may help us develop better methods to assess them taking into account particulars and generalities of sperm form and performance. Such tests could then become more reliable in estimations of the impact of cryopreservation or effect of changes in the environment and their relevance for fertility.

Sperm Differentiation: The Role of Trafficking of Proteins

“…At an evolutionary scale, selective forces could favor males that produce spermatozoa with certain shape or size if these traits are important determinants of their reproductive success [11]. When females mate with two or more males, competition between spermatozoa occurs in the female tract, and males with more sperm or sperm cells with faster or more efficient swimming may have advantages in fertilizing oocytes.…”

“…Females could also exercise some form of selection of one type of sperm over another or impose barriers that sperm need to overcome to reach the site of fertilization. These forces are collectively known as post-copulatory sexual selection [4,11]. An additional powerful selective force is represented by the mode of fertilization.…”

The social shape of sperm: Using an integrative machine-learning approach to examine sperm ultrastructure and collective motility

“…Despite sharing a common goal to reach and fertilize an ovum, sperm cells have undergone extensive evolutionary modification and are one of the most diverse cell types known [1]. This morphological variation is especially prevalent in mammals, in which sperm cells exhibit tremendous modifications in their compartmentalized and streamlined design [2][3][4], particularly in the size and shape of the sperm head [2, 5,6]. In most eutherian mammals, sperm heads are round or oval, but in rodents they are typically asymmetric and falciform, featuring one or more apical (which was not certified by peer review) is the author/funder.…”

Evolution of sperm morphology in a crustacean genus with fertilization inside an open brood pouch

“…The ejaculate size is known to evolve in response to the risk of sperm competition for egg fertilization, to cryptic female choice and to female receptacle size (i.e. the dilution effect) (Roldan, 2019). In contrast, while sperm are considered as one of the most taxonomically diverse and rapidly evolving cell types (Birkhead et al , 2009; Ramm et al , 2014), the understanding of the adaptive value of sperm morphology, such as length and shape, remains largely incomplete (Lüpold & Pitnick, 2018).…”