The organization,regulation,and biological functions of the synaptonemal complex

The synaptonemal complex(SC)is a meiosis-specific proteinaceous macromolecular structure that assembles between paired homologous chromosomes during meiosis in various eukaryotes.The SC has a highly conserved ultrastructure and plays critical roles in controlling multiple steps in meiotic recombination and crossover formation,ensuring accurate meiotic chromosome segregation.Recent studies in different organisms,facilitated by advances in super-resolution microscopy,have provided insights into the macromolecular structure of the SC,including the internal organization of the meiotic chromosome axis and SC central region,the regulatory pathways that control SC assembly and dynamics,and the biological functions exerted by the SC and its substructures.This review summarizes recent discoveries about how the SC is organized and regulated that help to explain the biological functions associated with this meiosis-specific structure.

The Synaptonemal Complex of Basal Metazoan Hydra:More Similarities to Vertebrate than Invertebrate Meiosis Model Organisms

The synaptonemal complex （SC） is an evolutionarily well-conserved structure that mediates chromosome synapsis during prophase of the first meiotic division. Although its structure is conserved, the characterized protein components in the current metazoan meiosis model systems （Drosophila melanogaster, Caenorhabditis elegans, and Mus musculus） show no sequence homology, challenging the question of a single evolutionary origin of the SC. However, our recent studies revealed the monophyletic origin of the mammalian SC protein components. Many of them being ancient in Metazoa and already present in the cnidarian Hydra. Remarkably, a comparison between different model systems disclosed a great similarity between the SC components of Hydra and mammals while the proteins of the ecdysozoan systems （D. rnelanogaster and C. elegans） differ significantly. In this review, we introduce the basal-branching metazoan species Hydra as a potential novel invertebrate model system for meiosis research and particularly for the investigation of SC evolution, function and assembly. Also, available methods for SC research in Hydra are summarized.

Karyotypes and recombination patterns of the Common Swift(Apus apus Linnaeus,1758) and Eurasian Hobby(Falco subbuteo Linnaeus, 1758)

Background: Meiotic recombination is an important source of genetic variability.Studies on mammals demonstrate a substantial interspecies variation in overall recombination rate, which is dependent mainly on chromosome(2 n) and chromosome arm number(FN).Bird karyotypes are very conservative with 2 n being about 78–82 and FN being 80–90 in most species.However, some families such as Apodidae(swifts) and Falconidae(falcons) show a substantial karyotypic variation.In this study, we describe the somatic and pachytene karyotypes of the male Common Swift(Apus apus) and the pachytene karyotype of the male Eurasian Hobby(Falco subbuteo) and estimate the overall number and distribution of recombination events along the chromosomes of these species.Methods: The somatic karyotype was examined in bone marrow cells.Pachytene chromosome spreads were prepared from spermatocytes of adult males.Synaptonemal complexes and mature recombination nodules were visualized with antibodies to SYCP3 and MLH1 proteins correspondingly.Results: The karyotype of the Common Swift consists of three metacentric, three submetacentric and two telocentric macrochromosomes and 31 telocentric microchromosomes(2 n = 78; FN = 90).It differs from the karyotypes of related Apodidae species described previously.The karyotype of the Eurasian Hobby contains one metacentric and 13 telocentric macrochromosomes and one metacentric and ten telocentric microchromosomes(2 n = 50; FN = 54) and is similar to that described previously in 2 n, but differs for macrochromosome morphology.Despite an about 40% difference in 2 n and FN, these species have almost the same number of recombination nodules per genome: 51.4 ± 4.3 in the swift and 51.1 ± 6.7 in the hobby.The distribution of the recombination nodules along the macrochromosomes was extremely polarized in the Common Swift and was rather even in the Eurasian Hobby.Conclusions: This study adds two more species to the short list of birds in which the number and distribution of recombination nodules have been examined.Our data confirm that recombination rate in birds is substantially higher than that in mammals, but shows rather a low interspecies variability.Even a substantial reduction in chromosome number does not lead to any substantial decrease in recombination rate.More data from different taxa are required to draw statistically supported conclusions about the evolution of recombination in birds.

Effects of Guhanyangshengjing Tablet on Testosterone Synthesis and Expression of SYCP3 in the Testis of Aging Male Rats

Objective: The objective of this article is to investigate the effect of Guhanyangshengjing Tablet (GT) on expression of synaptonemal complex protein 3 (SYCP3), a meiotic marker, in the testis tissue of aging male rats. Methods: Forty aging male rats were randomly assigned into 4 equal groups (n = 10 per group). Rats in each group were treated with GT at dose of 0 (control), 1.5 g/kg, 3.0 g/kg or 4.5 g/kg respectively by gavage daily for 30 days. At the end of the experiment, all animals were sacrificed and the blood samples were drawn to evaluate serum testosterone levels. The reproductive organs of each rat were taken and weighted. The right testis of each rat was removed for the analysis of intratesticular testosterone (ITT) concentrations, and the left one was used for immunohistochemical staining. Results: Compared with the control, reproductive organs’ weights, serum testosterone levels, ITT concentrations, quality of sperm, and expression of SYCP3 in the GT-treated groups were all improved in a dose-dependent manner. Conclusion: GT can improve testosterone synthesis and promote spermatogenesis simultaneously, indicating that GT is suitable for late-onset hypogonadism (LOH) patients with fertility requirements.

Meiotic Chromosome Behavior in a Human Male t(8;15)Carrier

Reciprocal translocation is one of the most common structural chromosomal rearrangements in human beings; it is widely recognized to be associated with male infertility. This association is mainly based on the abnormal chromosome behavior of the translocated chromosomes and sex chromosomes during meiosis prophase I in reciprocal translocation carriers. However, the underlying mechanisms are not completely known. Here we report a reciprocal translocation carrier of t（8;15）, who is oligozoospermic due to apoptosis of primary spermatocytes and to premature germ cell desquamation from seminiferous tubules. Further analysis showed abnormal synapsis and recombination frequency in this patient, indicating a connection between chromosome behavior and apoptosis of primary spermatocytes. We also compared these observations with recently reported findings on spermatogenesis defects in reciprocal translocation carriers, and discuss the possible mechanisms underlying both common and unique phenotypes of reciprocal translocations involving different chromosomes with the aim of further understanding the regulation of human spermatogenesis.

A practical reference for studying meiosis in the model ciliate Tetrahymena thermophila

Meiosis is a critical cell division program that produces haploid gametes for sexual reproduction.Abnormalities in meiosis are often causes of infertility and birth defects(e.g.,Down syndrome).Most organisms use a highly specialized zipper-like protein complex,the synaptonemal complex(SC),to guide and stabilize pairing of homologous chromosomes in meiosis.Although the SC is critical for meiosis in many eukaryotes,there are organisms that perform meiosis without a functional SC.However,such SC-less meiosis is poorly characterized.To understand the features of SC-less meiosis and its adaptive significance,the ciliated protozoan Tetrahymena was selected as a model.Meiosis research in Tetrahymena has revealed intriguing aspects of the regulatory programs utilized in its SC-less meiosis,yet additional efforts are needed for obtaining an in-depth comprehension of mechanisms that are associated with the absence of SC.Here,aiming at promoting a wider application of Tetrahymena for meiosis research,we introduce basic concepts and core techniques for studying meiosis in Tetrahymena and then suggest future directions for expanding the current Tetrahymena meiosis research toolbox.These methodologies could be adopted for dissecting meiosis in poorly characterized ciliates that might reveal novel features.Such data will hopefully provide insights into the function of the SC and the evolution of meiosis from a unique perspective.

SYP-5 regulates meiotic thermotolerance in Caenorhabditis elegans

Meiosis produces the haploid gametes required by all sexually reproducing organisms,occurring in specific temperature ranges in different organisms.However,how meiotic thermotolerance is regulated remains largely unknown.Using the model organism Caenorhabditis elegans,here,we identified the synaptonemal complex(SC)protein SYP-5 as a critical regulator of meiotic thermotolerance.syp-5-null mutants maintained a high percentage of viable progeny at 20℃ but produced significantly fewer viable progeny at 25℃,a permissive temperature in wild-type worms.Cytological analysis of meiotic events in the mutants revealed that while SC assembly and disassembly,as well as DNA double-strand break repair kinetics,were not affected by the elevated temperature,crossover designation,and bivalent formation were significantly affected.More severe homolog segregation errors were also observed at elevated temperature.A temperature switching assay revealed that late meiotic prophase events were not temperature-sensitive and that meiotic defects during pachytene stage were responsible for the reduced viability of syp-5 mutants at the elevated temperature.Moreover,SC polycomplex formation and hexanediol sensitivity analysis suggested that SYP-5 was required for the normal properties of the SC,and charge-interacting elements in SC components were involved in regulating meiotic thermotolerance.Together,these findings provide a novel molecular mechanism for meiotic thermotolerance regulation.