Post-Meiotic Intra-Testicular Sperm Senescence in a Wild Vertebrate

There is growing interest in sperm senescence, both in its underlying mechanisms and evolutionary consequences, because it can impact the evolution of numerous life history traits. Previous studies have documented various types of sperm senescence, but evidence of post-meiotic intra-testicular sperm senescence in wild animals is lacking. To assess such senescence, we studied within-season changes in sperm motility in the common toad (Bufo bufo), where males produce all sperm prior to the breeding season. We found that males exposed to experimentally induced re-hibernation at the start of the breeding season, that is to experimentally lowered metabolic rates, stored sperm of significantly higher motility than males that were kept under seminatural conditions without females throughout the breeding season. This finding indicates that re-hibernation slows normal rates of sperm ageing and constitutes the first evidence to our knowledge of post-meiotic intra-testicular sperm senescence in a wild vertebrate. We also found that in males kept in seminatural conditions, sperm motility was positively related to the number of matings a male achieved. Thus, our results suggest that post-meiotic intra-testicular sperm senescence does not have a genetically fixed rate and may be modulated by temperature and possibly by mating opportunities.     

Natural variation of the Y chromosome suppresses sex ratio distortion and modulates testis-specific gene expression in Drosophila simulans

X-linked sex-ratio distorters that disrupt spermatogenesis can cause a deficiency in functional Y-bearing sperm and a female-biased sex ratio. Y-linked modifiers that restore a normal sex ratio might be abundant and favored when a X-linked distorter is present. Here we investigated natural variation of Y-linked suppressors of sex-ratio in the Winters systems and the ability of these chromosomes to modulate gene expression in Drosophila simulans. Seventy-eight Y chromosomes of worldwide origin were assayed for their resistance to the X-linked sex-ratio distorter gene Dox. Y chromosome diversity caused males to sire ∼63% to ∼98% female progeny. Genome-wide gene expression analysis revealed hundreds of genes differentially expressed between isogenic males with sensitive (high sex ratio) and resistant (low sex ratio) Y chromosomes from the same population. Although the expression of about 75% of all testis-specific genes remained unchanged across Y chromosomes, a subset of post-meiotic genes was upregulated by resistant Y chromosomes. Conversely, a set of accessory gland-specific genes and mitochondrial genes were downregulated in males with resistant Y chromosomes. The D. simulans Y chromosome also modulated gene expression in XXY females in which the Y-linked protein-coding genes are not transcribed. The data suggest that the Y chromosome might exert its regulatory functions through epigenetic mechanisms that do not require the expression of protein-coding genes. The gene network that modulates sex ratio distortion by the Y chromosome is poorly understood, other than that it might include interactions with mitochondria and enriched for genes expressed in post-meiotic stages of spermatogenesis.     

Chlornaphazine and chlorambucil induce dominant lethal mutations in male mice

Two antineoplastic agents, chlornaphazine (CN) and chlorambucil (CHL), were tested for the induction of dominant lethal mutations in male mice. Both compounds are nitrogen mustard derivatives and have been shown to be genotoxic in a variety of organisms. CN was administered intraperitoneally to DBA/2J male mice at a dosage of 0, 500, 1000, or 1500 mg/kg body weight (bw). Immediately following treatment, each male was mated at 4-day intervals to two virgin C57BL/6J females. CHL was administered intraperitoneally to C3H/HeJ and DBA/2J males at a dosage of 0, 2.5, or 5.0 mg/kg bw. These males were mated at weekly intervals to two virgin T-stock females. CN and CHL clearly induced dominant lethal mutations. CN induced dominant lethal effects in all post-meiotic germ-cell stages of treated DBA males, with a clear dose-response relationship. The results with CHL-treated DBA males indicated that all post-meiotic germ-cell stages, except late-spermatids, were affected by CHL treatment, while in C3H males, CHL induced dominant lethal effects in all post-meiotic germ-cell stages. A dose-response relationship was also observed with CHL in C3H male mice. In the present experiments, regardless of the agent or the mouse strain used, spermatids appeared to be the germ-cell stage most sensitive to dominant lethal induction.     

The induction by X-rays of chromosome aberrations in male guinea-pigs, golden hamsters and rabbits. II. Properties of translocations induced in post-meiotic stages.

Translocations induced by X-rays in post-meiotic germ cells of male guinea-pigs, golden hamsters and rabbits were studied cytologically in the F1 sons of the irradiated males. The percentage of spermatocytes displaying multivalent configurations varied with the translocation, but the average percentage appeared to depend on the species: fewer quadrivalents were observed in hamster than in guinea-pig heterozygotes and most were recorded for rabbit heterozygotes. Chain quadrivalents were more abundant than ring quadrivalents at meiosis for the guinea-pig and hamster, in contrast to the mouse. Too few translocation heterozygotes were examined to determine which meiotic configuration was the more prevalent in the rabbit. In all three species, as in the mouse, translocations were found which caused male sterility, due to partial or complete failure of spermatogenesis, although most translocations caused semi-sterility. For these semi-sterile males both the frequency and time of embryonic death in the progeny appeared to be the same as in the mouse. It is concluded that similar types of chromosome aberrations are induced by X-rays in post-meiotic germ cells of male guinea-pigs, rabbits, golden hamsters and mice.     

Modification of radiation-induced genetic damage in Drosophila melanogaster male germ cells with nucleic acid precursors II. Effects on post-meiotic cells

Following the observation that the nucleoside pre-treatment reduced the radiation-induced dominant lethality in the post-meiotic germ cells, similar experiments were conducted using the same treatment conditions to study the influence of the nucleoside(s) pre-treatment on the radiation-induced (1.2 kR) incidence of sex-linked recessive lethals and translocation events in the post-meiotic male germ cells of 1-day-old D. melanogaster. The nucleoside pre-treatment reduced the translocation frequency (not statistically significant) and the lethal mutation frequency (statistically significant) in the post-meiotic cells (pre-injection DNA synthesis cells) especially in the mature sperms sampled in brood a (br a). The radio-protective effect of the nucleosides on the mature sperms was confirmed using 7-day-old virgin males and different radiation doses (2.4 kR and 3.6 kR).The frequency of lethal mutation was lowest when irradiation was preceded by the injection of an equimolar solution of thymidine (TdR), deoxyadenosine (AdR), deoxycytidine (CdR) and deoxyguanosine (GdR). However, when the nucleosides were injected after irradiation (within 10–30 min) there was no change in the yield of radiation-induced lethals.The possible mechanisms for the radioprotective action of the nucleosides in the post-meiotic germ cells such as (a) “protection” by a radiochemical action of nucleosides competing for short-lived radicals that might otherwise cause damage to DNA and (b) biochemical-physiological mechanisms such as metabolic events increasing the radioresistance of the cells, providing excess energy for repair or favoring and partaking in the DNA repair synthesis were discussed. Further studies were felt necessary to elucidate this phenomenon.     

A floral third whorl-specific marker gene in the dioecious species white campion is differentially expressed in mutants defective in stamen development

The CCLS4 gene of white campion is specifically expressed in anther epidermis and endothecium from pre- to post-meiotic stages. We report on a detailed in situ analysis of the gene's expression and show that it is a marker of the floral third whorl. The gene is expressed (1) in the anther (epidermis, parietal cells and the derived endothecium) in normally developing stamens and (2) in distinct sub-domains of third-whorl epidermis in mutants exhibiting aberrant states of parietal differentiation. Our results suggest that CCLS4 may fulfil different functions during pre- and post-meiotic anther development and reveal the complex role parietal cells may play during early stamen formation.     

Role of β-catenin in post-meiotic male germ cell differentiation

Though roles of β-catenin signaling during testis development have been well established, relatively little is known about its role in postnatal testicular physiology. Even less is known about its role in post-meiotic germ cell development and differentiation. Here, we report that β-catenin is highly expressed in post-meiotic germ cells and plays an important role during spermiogenesis in mice. Spermatid-specific deletion of β-catenin resulted in significantly reduced sperm count, increased germ cell apoptosis and impaired fertility. In addition, ultrastructural studies show that the loss of β-catenin in post-meiotic germ cells led to acrosomal defects, anomalous release of immature spermatids and disruption of adherens junctions between Sertoli cells and elongating spermatids (apical ectoplasmic specialization; ES). These defects are likely due to altered expression of several genes reportedly involved in Sertoli cell-germ cell adhesion and germ cell differentiation, as revealed by gene expression analysis. Taken together, our results suggest that β-catenin is an important molecular link that integrates Sertoli cell-germ cell adhesion with the signaling events essential for post-meiotic germ cell development and maturation. Since β-catenin is also highly expressed in the Sertoli cells, we propose that binding of germ cell β-catenin complex to β-catenin complex on Sertoli cell at the apical ES surface triggers a signaling cascade that regulates post-meiotic germ cell differentiation.     

Auxotrophic markers and the cytogenetics of Filobasidiella neoformans

In the sexual phase of Cryptococcus neoformans four post-meiotic nuclei give rise to the nuclei of four long chains of basidiospores. If there is a fixed relationship between one post-meiotic nucleus and one chain the result should be genetic homogeneity within single chains; random composition of chains would represent the opposite case. We tested for homogeneity in single chains from crosses of niacin-auxotrophs to the wild type by dissecting single chains of basidiospores from mature fruiting bodies. Out of 14 chains, 11 were homogeneous; the probability of obtaining this result by random processes was <0.01. These results suggest that mitotic products of a particular post-meiotic nucleus may be channeled into a particular basidiospore chain.