Sperm Gatekeeping: 3D Imaging Reveals a Constricted Entrance to Zebra Finch Sperm Storage Tubules

Fertility and Insemination Characteristics of Sperm Storage Tubules in Old Thai-Native Hens

We aimed to evaluate the effects of sperm concentration (150-250 × 106 spz/dose) and insemination frequency (once, twice, and thrice weekly) on fertility and sperm storage tubule (SST) characteristics. The SSTs were classified into five categories: namely, SSTs having an unscorable (SST1), empty (SST2), low (SST3), medium (SST4), and high (SST5) sperm count after insemination. The results showed that only insemination frequency affected the fertility rate (p < 0.05). The highest fertility was found in the thrice-weekly insemination group; however, this rate was not significantly different from that for the twice-weekly insemination group, except on day 7, while the once-weekly insemination group showed the lowest fertility rate (p < 0.05) from day four onward. On day 1, the SST characteristics showed no differences among the various insemination frequencies. On day 4, the SST2 and SST3 categories increased in the once-weekly insemination group (p < 0.05), while the SST4 and SST5 categories decreased compared to the twice- and thrice-weekly insemination groups (p < 0.05). On day 7, only the thrice-weekly insemination group maintained a level of SST5 category tubules like that measured on day 1 (p > 0.05). In summary, the insemination dose of 150 × 106 sperm was enough for fertilization, and thrice-weekly insemination was the appropriate frequency in old Thai native hens for maintaining a high sperm density in the SSTs throughout the week.

Turkey hen sperm storage tubule transcriptome response to artificial insemination and the presence of semen

Introduction: Sperm storage within the uterovaginal junction (UVJ) of avian species occurs in specialized structures termed sperm storage tubules (SSTs) and allows for prolonged storage of semen, though the molecular mechanisms involved in semen preservation are not well understood. Little work has been done examining how function of the SSTs is impacted by insemination and by semen present in the SSTs. Methods: Transcriptome analysis was performed on isolated SSTs from turkey hens receiving no insemination (control), sham-insemination, or semen-insemination at three timepoints (D1, D30, and D90 post-insemination). Bioinformatic and functional annotation analyses were performed using CLC Genomics Workbench, Database for Annotation, Visualization, and Integrated Discovery (DAVID), and Ingenuity Pathway Analysis (IPA). Pairwise comparisons and k-medoids cluster analysis were utilized to decipher differential expression profiles in the treatment groups. Results: The SST transcriptome of the semen inseminated group exhibited the greatest differences within the group, with differences detectable for up to 90 days post insemination, while control and sham-inseminated groups were more similar. In the semen-inseminated samples, upregulation of pathways relating to classical and non-classical reproductive signaling, cytoskeletal remodeling, physiological parameters of the local UVJ environment, and cellular metabolism was observed. In the sham-inseminated samples, upregulation of immune pathways and non-reproductive endocrine hormones was observed. Discussion: This work provides insights into the molecular level changes of the SST in response to insemination as well as to the presence of semen. Results from this study may have direct implications on fertility rates as well as potential strategies for avian semen cryopreservation protocols.

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulations of high promiscuity (10 mates per female) showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2-3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Furthermore, when promiscuity is low, disruptive selection on male traits will occur under much more limited circumstances (i.e. only with higher among-female variation) than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.

Flagellum tapering and midpiece volume in songbird spermatozoa

In contrast to numerous studies on spermatozoa length, relatively little work focuses on the width of spermatozoa, and particularly the width of the midpiece and flagellum. In flagellated spermatozoa, the flagellum provides forward thrust while energy may be provided via mitochondria in the midpiece and/or through glycolysis along the flagellum itself. Longer flagella may be able to provide greater thrust but may also require stronger structural features and more or larger mitochondria to supply sufficient energy. Here, we use scanning electron microscopy to investigate the ultrastructure of spermatozoa from 55 passerine species in 26 taxonomic families in the Passerides infraorder. Our data confirm the qualitative observation that the flagellum tapers along its length, and we show that longer flagella are wider at the neck. This pattern is similar to mammals, and likely reflects the need for longer cells to be stronger against shearing forces. We further estimate the volume of the mitochondrial helix and show that it correlates well with midpiece length, supporting the use of midpiece length as a proxy for mitochondrial volume, at least in between-species studies where midpiece length is highly variable. These results provide important context for understanding the evolutionary correlations among different sperm cell components and dimensions.

Differential gene expression analysis related to sperm storage in spermathecas of Amphioctopus fangsiao

Sperm storage in the female body is an important strategy in animal reproductive behavior. Amphioctopus fangsiao is an economically important cephalopod that has a sperm storage period of up to seven months. There are few studies concerning the mechanism of sperm storage in A. fangsiao. In this study, we performed transcriptome gene expression profiling of the oviductal glands at different phases (presence and absence of sperm storage). In total, 7943 differentially expressed genes (DEGs) comprising 4737 upregulated and 3206 downregulated genes were identified. GO and KEGG enrichment analyses were used to search for sperm storage-related genes. A protein interaction network was constructed to examine the interactions between genes. Nineteen genes associated with immunity, apoptosis, and autophagy were obtained and verified by qRT-PCR. This is the first comprehensive analysis of sperm storage-related genes in A. fangsiao. The results provide basic insights into the complex sperm storage mechanism of A. fangsiao.

Transcriptome analysis of inseminated sperm storage tubules throughout the duration of fertility in the domestic turkey, Meleagris gallopavo

Sperm storage tubules (SST) are specialized invaginations of the oviductal epithelium that permit avian species to store spermatozoa for extended periods of time, without compromising sperm fertilization capacity. The molecular and physiological mechanisms behind sperm storage tubule differentiation, sperm protection, and regression remain largely unknown, but most likely have potential implications for substantially improving hen fertility, sperm storage, and semen cryopreservation in commercial poultry species. RNA sequencing was performed on sperm storage tubules isolated from the epithelium of the uterovaginal junction (UVJ) from hens at d 1, 7, 30, 60, and 90 postinsemination (n = 4 per timepoint). Read mapping and differential expression analysis were performed using CLC Genomics Workbench. A total of 2,340 differentially expressed genes were subjected to pathway analysis through Ingenuity Pathway Analysis (IPA). Through functional annotation of differentially expressed genes during early, peak, and late egg production, novel insights regarding the role of innate and acquired immune response to sperm, lipid synthesis and transfer, steroid hormone signalling, cytoskeletal reorganization, and regulation of ion homeostasis in SST were obtained. Additionally, potential pathways were identified that could be involved with suppressing sperm motility while sperm reside within the SST. Upstream analysis identified potential regulatory roles for 18 upstream regulators that could modulate sperm storage tubule function, including suppression of sperm motility. Understanding sperm storage tubule function throughout the laying cycle, especially with regards to sperm preservation may allow for the development of industry-based protocols for semen storage and cryopreservation that mimic the sperm preservation capabilities of SST and improve fertility.

Physiological factors influencing female fertility in birds

Fertility is fundamental to reproductive success, but not all copulation attempts result in a fertilized embryo. Fertilization failure is especially costly for females, but we still lack a clear understanding of the causes of variation in female fertility across taxa. Birds make a useful model system for fertility research, partly because their large eggs are easily studied outside of the female's body, but also because of the wealth of data available on the reproductive productivity of commercial birds. Here, we review the factors contributing to female infertility in birds, providing evidence that female fertility traits are understudied relative to male fertility traits, and that avian fertility research has been dominated by studies focused on Galliformes and captive (relative to wild) populations. We then discuss the key stages of the female reproductive cycle where fertility may be compromised, and make recommendations for future research. We particularly emphasize that studies must differentiate between infertility and embryo mortality as causes of hatching failure, and that non-breeding individuals should be monitored more routinely where possible. This review lays the groundwork for developing a clearer understanding of the causes of female infertility, with important consequences for multiple fields including reproductive science, conservation and commercial breeding.

How biases in sperm storage relate to sperm use during oviposition in female yellow dung flies

Precise mechanisms underlying sperm storage and utilization are largely unknown, and data directly linking stored sperm to paternity remain scarce. We used competitive microsatellite PCR to study the effects of female morphology, copula duration and oviposition on the proportion of stored sperm provided by the second of two copulating males (S2) in Scathophaga stercoraria (Diptera: Scathophagidae), the classic model for sperm competition studies. We genotyped all offspring from potentially mixed-paternity clutches to establish the relationship between a second male’s stored sperm (S2) and paternity success (P2). We found consistent skew in sperm storage across the three female spermathecae, with relatively more second-male sperm stored in the singlet spermatheca than in the doublet spermathecae. S2 generally decreased with increasing spermathecal size, consistent with either heightened first-male storage in larger spermathecae, or less efficient sperm displacement in them. Additionally, copula duration and several two-way interactions influenced S2, highlighting the complexity of postcopulatory processes and sperm storage. Importantly, S2 and P2 were strongly correlated. Manipulation of the timing of oviposition strongly influenced observed sperm-storage patterns, with higher S2 when females laid no eggs before being sacrificed than when they oviposited between copulations, an observation consistent with adaptive plasticity in insemination. Our results identified multiple factors influencing sperm storage, nevertheless suggesting that the proportion of stored sperm is strongly linked to paternity (i.e., a fair raffle). Even more detailed data in this vein are needed to evaluate the general importance of sperm competition relative to cryptic female choice in postcopulatory sexual selection.