Methodological considerations for examining the relationship between sperm morphology and motility

Abstract

Sperm cells of all taxa share a common goal to reach and fertilize an ovum, yet sperm are one of the most diverse cell types in nature. While the structural diversity of these cells is well recognized, the functional significance of variation in sperm design remains elusive. An important function of spermatozoa is a need to migrate toward the ova, often over long distances in a foreign environment, which may include a complex and hostile female reproductive tract. Several comparative and experimental studies have attempted to address the link between sperm morphology and motility, yet the conclusions drawn from these studies are often inconsistent, even within the same taxa. Much of what we know about the functional significance of sperm design in internally fertilizing species has been gleaned from in vitro studies, for which experimental parameters often vary among studies. We propose that discordant results from these studies are in part due to a lack of consistency of methods, conditions that do not replicate those of the female reproductive tract, and the overuse of simple linear measures of sperm shape. Within this review, we provide a toolkit for imaging, quantifying, and analyzing sperm morphology and movement patterns for in vitro studies and discuss emerging approaches. Results from studies linking morphology to motility enhance our understanding of the evolution of adaptive sperm traits and the mechanisms that regulate fertility, thus offering new insights into methods used in assisted reproductive technologies in animal science, conservation and public health.

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

“…While sperm morphological diversity is well documented and recognized [e.g., 1,12], identifying and quantifying the specific structural attributes that confer functional benefits has proven challenging, particularly for sperm cells with complex shapes [7,11]. Using an integrative methodical and analytical approach that combines traditional morphometry and machine learning, we characterized sperm head shape variation and investigated its role in collective motility in the Peromyscus lineage.…”

“…The copyright holder for this preprint this version posted December 7, 2020. ; https://doi.org/10.1101/2020.12.05.413120 doi: bioRxiv preprint hooks that vary in their length and curvature [3,[7][8][9]. For sperm head designs that are complex, identifying and quantifying informative morphological features and, thus, their functional significance has been an ongoing challenge across taxa [10][11][12][13][14].…”

Post-copulatory sexual selection is associated with sperm aggregate quality inPeromyscusmice

“…When sperm heads and flagella are not oriented in the same direction, the cells within an aggregate are expected to exert opposing forces on one another, thereby reducing the overall motility of the group (Fisher et al 2014; Pearce et al 2018). Similarly, the inclusion of immotile or morphologically abnormal sperm, which typically display reduced motility (reviewed in Hook and Fisher 2020), is expected to reduce overall group kinematics. Indeed, we found that aggregates with misaligned, immotile and/or morphologically abnormal sperm were slower than aggregates without these abnormalities in all but one species.…”

Sperm bauplan and function and underlying processes of sperm formation and selection

“…Very few studies exist in which shape and motility parameters have been combined to characterize sperm subpopulations (391) and to the best of our knowledge there are no reports yet on links between a combined analysis of shape and motility subpopulations and sperm fertility. As a cautionary note, it is perhaps important to bear in mind that diversity in sperm morphology and kinetic traits (or lack of them) may be related to methodologies employed and there is a riks of generating artifacts during analyses (reviewed in 182,351,390,454). In any case, the question that arises is why males produce not only vast numbers of spermatozoa but, also, these heterogenous populations if only a few sperm cells would be successful at fertilization.…”