Abstract
Manuscript provides insights into the biology of long-lived plants, different from Arabidopsis, tomato or grass species that are widely studied. In the European larch the diplotene stage lasts approximately 5 months and it is possible to divide it into several substages and to observe each of them in details. The diplotene stage is a period of intensive microsporocyte growth associated with the synthesis and accumulation of different RNA and proteins. Larch microsporocytes display changes in chromatin morphology during this stage, alternating between 4 short stages of chromatin condensation (contraction) and 5 longer diffusion (relaxation) stages. The occurrence of a diplotene diffusion stage has been observed in many plant species. Interestingly, they have also been observed during spermiogenesis and oogenesis in animals. The aim of this study was to examine whether chromatin relaxation during the diplotene is accompanied by the synthesis and maturation of mRNA. The results reveal a correlation between the diffusion and chromatin decondensation, transcriptional activity. We also found decreasing amount of poly(A) mRNA synthesis in the consecutive diffusion stages. During the early diffusion stages, mRNA is intensively synthesized. In the nuclei large amounts of RNA polymerase II, and high levels of snRNPs were observed. In the late diffusion stages, the synthesized mRNA is not directly subjected to translation but it is stored in the nucleus, and later transported to the cytoplasm and translated. In the last diffusion stage, the level of poly(A) RNA is low, but that of splicing factors is still high. It appears that the mRNA synthesized in early stages is used during the diplotene stage and is not transmitted to dyad and tetrads. In contrast, splicing factors accumulate and are most likely transmitted to the dyad and tetrads, where they are used after the resumption of intense transcription. Similar meiotic process were observed during oogenesis in animals. This indicates the existence of an evolutionarily conserved mechanism of chromatin-based regulation of gene expression during meiotic prophase I.
Highlights
Meiosis is an important step in sexual reproduction
Our analysis indicates that 70–95% of the cells collected from one harvest were similar in size and chromatin morphology
Strong chromatin condensation during the pachytene stage was followed by chromatin diffusion at the beginning of the diplotene stage, but we observed 4 transient stages in which chromatin was more compact that alternated with 4longer diffuse stages, of various lengths (S1–4 Figs.)
Summary
Meiosis is an important step in sexual reproduction. It is a specialised cell division that generates four haploid cells from a diploid parent cell after a single round of DNA replication and two consecutive rounds of nuclear division. The most complex phase of meiotic division is meiotic prophase I During this tage, nuclear organisation dramatically changes. Meiosis stages are associated with a meiosis-specific organisational structure known as synaptonemal complex (SC) formation, pairing and recombination between homologous chromosomes. The diplotene stage lasts for approximately 4–5 months in various larch species, and it includes several chromatin relaxation stages. The occurrence of a diffusion stage has been observed in many plant species [5,6,7,8,9,10,11,12,13].recent studies in L. leptolepis microsporocytes have shown that chromosomes undergo several gradual and major organisational changes during the diplotene stage [14]. The diffusion phase during diplotene has been observed during spermiogenesis and oogenesis in animals [15,16,17,18,19]
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