Sperm structure and motility in the eusocial naked mole-rat, Heterocephalus glaber: a case of degenerative orthogenesis in the absence of sperm competition?

Abstract

BackgroundWe have studied sperm structure and motility in a eusocial rodent where reproduction is typically restricted to a single male and behaviourally dominant queen. Males rarely compete for access to the queen during her estrus cycle, suggesting little or no role for sperm competition.ResultsOur results revealed an atypical mammalian sperm structure with spermatozoa from breeding, subordinate and disperser males being degenerate and almost completely lacking a "mammalian phylogenetic stamp". Sperm structure is characterized by extreme polymorphism with most spermatozoa classified as abnormal. Sperm head shapes include round, oval, elongated, lobed, asymmetrical and amorphous. At the ultrastructural level, the sperm head contains condensed to granular chromatin with large open spaces between the chromatin. Nuclear chromatin seems disorganized since chromatin condensation is irregular and extremely inconsistent. The acrosome forms a cap (ca 35%) over the anterior part of the head. A well defined nuclear fossa and neck with five minor sets of banded protein structures are present. The midpiece is poorly organized and contains only 5 to 7 round to oval mitochondria. The flagellar pattern is 9+9+2. A distinct degenerative feature of the tail principal piece is the absence of the fibrous sheath. Only 7% motile spermatozoa were observed which had exceptionally slow swimming speeds.ConclusionIn this species, sperm form has simplified and degenerated in many aspects and represents a specialised form of degenerative orthogenesis at the cellular level.

Inhibitory effect of <i>Taraxacum officinale</i> L (Compositae) aqueous root extract on spermatogenesis

“…Sperms with rapid swimming abilities and normal morphology have a fertilizing advantage over slower abnormal sperms [23]. T. officinale treatment reduced sperm parameters causing a reduction in the fertility rate and the number of fetuses/pregnant female which is in agreement with previously published results of Calendula officinalis (Compositae) administration [24].…”

Intramembranal disulfide cross-linking elucidates the super-quaternary structure of mammalian CatSpers

“…Naked mole rat is also missing Disulfide 3 cysteine Mm1:C362, and Disulfide 4 cysteine Mm3:C96, implying defects in subunit arrangement, polymerization, and cross-linking of rows in that species. Indeed, the swim speed for naked mole rat sperm has been quantified and characterized as “very slow”, and the slowness was attributed to the absence of sperm competition in that eusocial species [20]. Horseshoe bat has lost the proposed tetramer determinant Mm3:C197 from Disulfide 2, suggesting an impact on sperm motility.…”

Small and variable sperm sizes suggest low sperm competition despite multiple paternity in a lekking suboscine bird

“…The percentage of abnormal sperm varies: for example, 5-25% in ratites (du Plessis and Soley 2011) and ,25% in mammals, but with examples of extremely high levels of abnormal sperm (85-95%) despite normal fertilization in both naked mole rats and humans, in which sperm competition is low (van der Horst et al 2011). In fairy-wrens, the proportion of abnormal sperm varied with the level of sperm competition among species (5-30%; Rowe and Pruett-Jones 2011).…”