High-resolution crossover mapping reveals similarities and differences of male and female recombination in maize.

Meiotic crossovers (COs) are not uniformly distributed across the genome. Factors affecting this phenomenon are not well understood. Although many species exhibit large differences in CO numbers between sexes, sex-specific aspects of CO landscape are particularly poorly elucidated. Here, we conduct high-resolution CO mapping in maize. Our results show that CO numbers as well as their overall distribution are similar in male and female meioses. There are, nevertheless, dissimilarities at local scale. Male and female COs differ in their locations relative to transcription start sites in gene promoters and chromatin marks, including nucleosome occupancy and tri-methylation of lysine 4 of histone H3 (H3K4me3). Our data suggest that sex-specific factors not only affect male-female CO number disparities but also cause fine differences in CO positions. Differences between male and female CO landscapes indicate that recombination has distinct implications for population structure and gene evolution in male and in female meioses.

The plant-specific protein FEHLSTART controls male meiotic entry, initializing meiotic synchronization in Arabidopsis.

Meiosis marks the transition from the sporophyte to the gametophyte generation in the life cycle of flowering plants, and creates genetic variations through homologous recombination. In most flowering plants, meiosis is highly synchronized within each anther, which is significant for efficient fertilization. To date, little is known about the molecular mechanisms of entry into meiosis and exit from it, and only a few genes in Arabidopsis have been characterized with a role in regulating meiotic progression. In this study, we report the functional characterization of a plant-specific basic helix-loop-helix (bHLH) protein, FEHLSTART (FST), a defect in which leads to premature meiotic entry and asynchronous meiosis, and results in decreased seed yield. Investigation of the time course of meiosis showed that the onset of leptotene, the first stage of prophase I, frequently occurred earlier in fst-1 than in the wild type. Asynchronous meiosis followed, which could manifest in the disruption of regular spindle structures and symmetric cell divisions in fst-1 mutants during the meiosis I/II transition. In accordance with frequently accelerated meiotic entry, whole-transcriptome analysis of fst-1 anthers undergoing meiosis revealed that 19 circadian rhythm genes were affected and 47 pollen-related genes were prematurely expressed at a higher level. Taken together, we propose that FST is required for normal meiotic entry and the establishment of meiotic synchrony.

The transcriptome landscape of early maize meiosis.

BACKGROUND: A major step in the higher plant life cycle is the decision to leave the mitotic cell cycle and begin the progression through the meiotic cell cycle that leads to the formation of gametes. The molecular mechanisms that regulate this transition and early meiosis remain largely unknown. To gain insight into gene expression features during the initiation of meiotic recombination, we profiled early prophase I meiocytes from maize (Zea mays) using capillary collection to isolate meiocytes, followed by RNA-seq. RESULTS: We detected ~2,000 genes as preferentially expressed during early meiotic prophase, most of them uncharacterized. Functional analysis uncovered the importance of several cellular processes in early meiosis. Processes significantly enriched in isolated meiocytes included proteolysis, protein targeting, chromatin modification and the regulation of redox homeostasis. The most significantly up-regulated processes in meiocytes were processes involved in carbohydrate metabolism. Consistent with this, many mitochondrial genes were up-regulated in meiocytes, including nuclear- and mitochondrial-encoded genes. The data were validated with real-time PCR and in situ hybridization and also used to generate a candidate maize homologue list of known meiotic genes from Arabidopsis. CONCLUSIONS: Taken together, we present a high-resolution analysis of the transcriptome landscape in early meiosis of an important crop plant, providing support for choosing genes for detailed characterization of recombination initiation and regulation of early meiosis. Our data also reveal an important connection between meiotic processes and altered/increased energy production.

BRCA2 is a mediator of RAD51- and DMC1-facilitated homologous recombination in Arabidopsis thaliana.

• Mutations in the breast cancer susceptibility gene 2 (BRCA2) are correlated with hereditary breast cancer in humans. Studies have revealed that mammalian BRCA2 plays crucial roles in DNA repair. Therefore, we wished to define the role of the BRCA2 homologs in Arabidopsis in detail. • As Arabidopsis contains two functional BRCA2 homologs, an Atbrca2 double mutant was generated and analyzed with respect to hypersensitivity to genotoxic agents and recombination frequencies. Cytological studies addressing male and female meiosis were also conducted, and immunolocalization was performed in male meiotic prophase I. • The Atbrca2 double mutant showed hypersensitivity to the cross-linking agent mitomycin C and displayed a dramatic reduction in somatic homologous recombination frequency, especially after double-strand break induction. The loss of AtBRCA2 also led to severe defects in male meiosis and development of the female gametophyte and impeded proper localization of the synaptonemal complex protein AtZYP1 and the recombinases AtRAD51 and AtDMC1. • The results demonstrate that AtBRCA2 is important for both somatic and meiotic homologous recombination. We further show that AtBRCA2 is required for proper meiotic synapsis and mediates the recruitment of AtRAD51 and AtDMC1. Our results suggest that BRCA2 controls single-strand invasion steps during homologous recombination in plants.