Lack of alterations on meiotic chromosomes and morphological characteristics of male germ cells in mice exposed to a 60 Hz and 2.0 mT magnetic field

The effect of in vivo exposure of mice to a 60 Hz sinusoidal magnetic field (MF) at 2.0 mT on male germ cells was studied. The cytological endpoints measured included meiotic chromosome aberrations in spermatocytes and sperm morphology. Three independent experiments were carried out: (a) animals exposed for 72 h, (b) 10 days/8 h daily, and (c) 72 h exposure to MF plus 5 mg/kg of Mitomycin-C. No statistically significant differences indicative of MF effects were observed between MF exposed and control animals. In addition, an opposite effect between MF exposure and Mitomycin-C treatment in terms of chromosomal aberrations and sperm morphology was observed.     

Recombination and loss of complementation: a more than two-fold cost for parthenogenesis

Certain types of asexual reproduction lead to loss of complementation, that is unmasking of recessive deleterious alleles. A theoretical measure of this loss is calculated for apomixis, automixis and endomitosis in the cases of diploidy and polyploidy. The effect of the consequent unmasking of deleterious recessive mutations on fitness is also calculated. Results show that, depending on the number of lethal equivalents and on the frequency of recombination, the cost produced by loss of complementation after few generations of asexual reproduction may be greater than the two-fold cost of meiosis. Maintaining complementation may, therefore, provide a general short-term advantage for sexual reproduction. Apomixis can replace sexual reproduction under a wide range of parameters only if it is associated with triploidy or tetraploidy, which is consistent with our knowledge of the distribution of apomixis.     

A role for the mismatch repair system during incipient speciation in Saccharomyces

The cause of reproductive isolation between biological species is a major issue in the field of biology. Most explanations of hybrid sterility require either genetic incompatibilities between nascent species or gross physical imbalances between their chromosomes, such as rearrangements or ploidy changes. An alternative possibility is that genomes become incompatible at a molecular level, dependent on interactions between primary DNA sequences. The mismatch repair system has previously been shown to contribute to sterility in a hybrid between established yeast species by preventing successful meiotic crossing-over leading to aneuploidy. This system could also promote or reinforce the formation of new species in a similar manner, by making diverging genomes incompatible in meiosis. To test this possibility we crossed yeast strains of the same species but from diverse historical or geographic sources. We show that these crosses are partially sterile and present evidence that the mismatch repair system is largely responsible for this sterility.     

Meiotic state of bovine oocytes is regulated by interactions between cAMP,cumulus, and granulosa

Bovine oocytes are arrested at the prophase of first meiotic cell cycle. Meiosis resumes in oocytes of pre-ovulatory follicles upon LH surge. However, oocytes from secondary follicles spontaneously resume meiosis in the absence of hormones if removed from the follicle and cultured in vitro. The nature of meiotic arrestor in bovine follicles is poorly understood. In this study we investigated the role of cell-cell interactions between granulosa and cumulus cells and the oocyte in mediating maintenance of meiotic arrest by cAMP. We sorted oocytes as granulosa-cumulus oocyte complexes (GCOC) if surrounded with cumulus cells attached to a large granulosa investment or cumulus oocytes complexes (COC) if surrounded with cumulus cells only and investigated the role cAMP in maintenance of meiotic arrest in these oocytes under various conditions. In hormone- and serum-free medium both GCOC and COC enclosed oocytes resumed meiosis. When [cAMP](i) was elevated with addition of invasive adenylate cyclase (iAC) GCOC enclosed oocytes were maintained in the prophase with intact germinal vesicle (GV) while COC enclosed oocytes underwent GV breakdown (GVBD). iAC elevated [cAMP](i) in both types of oocytes to the same level. If oocytes were liberated from the cumulus and granulosa cells, they re-initiated meiosis in serum and hormone free medium, but remained in the GV stage if iAC was added to the medium. Untreated GCOC and COC enclosed oocytes extruded first polar body at the same frequency in hormone-supplemented media. GCOC and COC enclosed oocytes but not denuded oocytes (DO) cultured without somatic cells acquired developmental competence if cultured in hormone-containing medium. It is concluded that maintenance of meiotic arrest is regulated by the interplay of [cAMP](i), and cumulus and granulosa cells.     

H2AX regulates meiotic telomere clustering

The histone H2A variant H2AX is phosphorylated in response to DNA double-strand breaks originating from diverse origins, including dysfunctional telomeres. Here, we show that normal mitotic telomere maintenance does not require H2AX. Moreover, H2AX is dispensable for the chromosome fusions arising from either critically shortened or deprotected telomeres. However, H2AX has an essential role in controlling the proper topological distribution of telomeres during meiotic prophase I. Our results suggest that H2AX is a downstream effector of the ataxia telangiectasia-mutated kinase in controlling telomere movement during meiosis.     

De novo balanced translocation (2;10)(q24;q22) associated with mental retardation

We report a case of a reciprocal translocation between the long arms of the 2 and 10 chromosomes observed in a 14-year-old male with mild mental impairment, compulsive and obsessive behavior. The apparently balanced translocation was characterized by fluorescence in situ hybridization and the karyotype was 46, XY, t(2;10)(q24;q22). The way by balanced chromosomal translocations can lead to a disease phenotype are reviewed and discussed.     

Egg-to-embryo transition is driven by differential responses to Ca(2+) oscillation number

Ca(2+) oscillations and signaling represent a basic mechanism for controlling many cellular events. Activation of mouse eggs entrains a temporal series of Ca(2+)-dependent events that include cortical granule exocytosis, cell cycle resumption with concomitant decreases in MPF and MAP kinase activities, and recruitment of maternal mRNAs. The outcome is a switch in cellular differentiation, i.e., the conversion of the egg into the zygote. By activating mouse eggs with experimentally controlled and precisely defined Ca(2+) transients, we demonstrate that each of these events is initiated by a different number of Ca(2+) transients, while their completion requires a greater number of Ca(2+) transients than for their initiation. This combination of differential responses to the number of Ca(2+) transients provides strong evidence that a single Ca(2+) transient-driven signaling system can initiate and drive a cell into a new developmental pathway, as well as can account for the temporal sequence of cellular changes associated with early development.     

The second meiosis occurs in cytochalasin D-treated eggs of Corbicula leana even though it is not observed in control androgenetic eggs because the maternal chromosomes and centrosomes are extruded at first meiosis

The hermaphroditic freshwater clam Corbicula leana reproduces by androgenesis. In the control (androgenetic development), all maternal chromosomes and maternal centrosomes at the meiotic poles were extruded as the two first polar bodies, and subsequently, second meiosis did not occur. But, in C. leana eggs treated with cytochalasin D (CD) to inhibit polar body extrusion, the second meiosis occurred. At metaphase-I, the spindle showed the typical bipolar structure and two spheroid centrosomes were located at its poles. All the maternal chromosomes were divided at anaphase-I, but they were not extruded as polar bodies due to the effects of CD. After completion of first meiosis, the maternal centrosomes split into four. At the second meiosis, twin or tetrapolar spindles were formed and two groups of maternal chromosomes divided into four sets of chromosomes. After the second meiosis, the spindle disassociated and the four maternal centrosomes disappeared. Four groups of maternal chromosomes transformed into the four female pronuclei. Male and female pronuclei became metaphase chromosomes of the first mitosis. The present study clearly indicates that typical meiosis systems still proceed in androgenetic triploid C. leana. We conclude that the androgenetic form may have arisen from the meiotic form.