The telomere bouquet is a hub where meiotic double-strand breaks, synapsis, and stable homolog juxtaposition are coordinated in the zebrafish,Danio rerio

Abstract

The telomere bouquet is a hub where meiotic double-strand breaks, synapsis, and stable 1 homolog juxtaposition are coordinated in the zebrafish, Danio rerio Abstract 7Meiosis is a cellular program that generates haploid gametes for sexual reproduction. While 8 chromosome events that contribute to reducing ploidy (homologous chromosome pairing, 9 synapsis, and recombination) are well conserved, their execution varies across species and even 10 between sexes of the same species. The telomere bouquet is a conserved feature of meiosis that 11 was first described nearly a century ago, yet its role is still debated. Here we took advantage of the 12 prominent telomere bouquet in zebrafish, Danio rerio, and super-resolution microscopy to show 13 that axis morphogenesis, synapsis, and the formation of double-strand breaks (DSBs) all take place 14 within the immediate vicinity of telomeres. We established a coherent timeline of events and tested 15 the dependence of each event on the formation of Spo11-induced DSBs. First, we found that the 16 axis protein Sycp3 loads adjacent to telomeres and extends inward, suggesting a specific feature 17 common to all telomeres seeds the development of the axis. Second, we found that newly formed 18 axes near telomeres engage in presynaptic co-alignment by a mechanism that depends on DSBs, 19 even when stable juxtaposition of homologous chromosomes at interstitial regions is not yet 20 evident. Third, we were surprised to discover that ~30% of telomeres in early prophase I engage 21 in associations between two or more chromosome ends and these interactions decrease in later 22 stages. Finally, while pairing and synapsis were disrupted in both spo11 males and females, their 23 reproductive phenotypes were starkly different; spo11 mutant males failed to produce sperm while 24 females produced offspring with severe developmental defects. Our results support zebrafish as an 25 important vertebrate model for meiosis with implications for differences in fertility and genetically 26 derived birth defects in males and females. 28Author Summary 29Inherent to reproduction is the transmission of genetic information from one generation to the next. 30In sexually reproducing organisms, each parent contributes an equal amount of genetic 31 information, packaged in chromosomes, to the offspring. Diploid organisms, like humans, have 32 two copies of every chromosome, while their haploid gametes (e.g. eggs and sperm) have only 33 one. This reduction in ploidy depends on the segregation of chromosomes during meiosis, 34 resulting in gametes with one copy of each chromosome. Missegregation of the chromosomes in 35 the parents leads to abnormal chromosome numbers in the offspring, which is usually lethal or has 36 detrimental developmental effects. While it has been known for over a century that homologous 37 chromosomes pair and recombine to facilitate proper segregation, how homologs find their 38 partners has remained elusive. A structure that has been central to the discussion of homolog 39 pairing is...

Parthenogenesis as a solution to hybrid sterility: the mechanistic basis of meiotic distortions in clonal and sterile hybrids

“…Such stages are characterized by preliminary Double Strand Break (DSB)-independent pairing that restricts the searching area for homologous recognition and subsequent DSB formation and co-alignment [30]. In cypriniform fishes, the homology search is initiated in subtelomeric regions, followed by SYCP3 upload and zipping towards the interstitial chromosomal segments [31]. In case of sufficient homology between two chromosomes, zipping is finalized by uploading of SYCP1 protein accomplishing bivalent formation [31].…”

“…In cypriniform fishes, the homology search is initiated in subtelomeric regions, followed by SYCP3 upload and zipping towards the interstitial chromosomal segments [31]. In case of sufficient homology between two chromosomes, zipping is finalized by uploading of SYCP1 protein accomplishing bivalent formation [31]. It is therefore possible that low homology between orthologous chromosomes of contemporary species prevented sufficient pre-DSB coalignment in their hybrids, which instead possessed ectopic recombinational intermediates between similar sites of nonhomologous chromosomes.…”

Bloom syndrome helicase contributes to germ line development and longevity in zebrafish

“…Thus, similarly to blm -/mutants, both sex reversal and male subfertility can be observed in the mutants of the genes that encode these factors (Botthof et al, 2017;Ramanagoudr-Bhojappa et al, 2018;. In contrast, only subfertility was observed in the mutants of hsf5, mlh1 and spo11 all of which are involved in the meiotic process (Blokhina et al, 2019;Feitsma et al, 2007;Saju et al, 2018) (Figure 5A). During the early stages of ontogenesis vasa expressing primordial germ cells (PGC) proliferate and migrate from their initial positions adjacent to the yolk syncytial layer to the gonadal mesoderm.…”

“…Therefore, we sought to understand whether the lack of spermatids in the testis of blm −/− males is due to defective meiosis. Labeling DSBs with anti-γ-H2AX suggests that unlike in spo11 mutants (Blokhina et al, 2019), in blm −/− testes DSB formation is not affected (Supplementary Fig. 3).…”

“…3). We examined meiosis using an antibody specific for Sycp3, a component of the synaptonemal complex, previously shown to accumulate during the meiotic prophase (Blokhina et al, 2019). While in the testes of wild-type fish spermatids showed a typical mix of Sycp3 staining patterns, in those of blm −/− mutant individuals nuclei either displayed Sycp3 patterns typical for leptotene, or showed an aberrant accumulation of Sycp3 in the nuclear periphery (Figure 4A-C).…”