Apomictic plants (reproducing via asexual seeds), unlike sexual individuals, avoid meiosis and egg cell fertilization. Consequently, apomixis is very important for fixing maternal genotypes in the next plant generations. Despite the progress in the study of apomixis, molecular and genetic regulation of the latter remains poorly understood. So far APOLLO (Aspartate Glutamate Aspartate Aspartate histidine exonuclease) is the only described gene associated with apomixis in Boechera species. The centromere-specific histone H3 variant encoded by CENH3 gene is essential for cell division. Mutations in CENH3 disrupt chromosome segregation during mitosis and meiosis since the attachment of spindle microtubules to a mutated form of the CENH3 histone fails. This paper presents in silico characteristic of APOLLO and CENH3 genes, which may affect apomixis. Also, in this research we characterize the structure of CENH3, study expression levels of CENH3 and APOLLO in gynoecium/siliques of the natural diploid apomictic and sexual Boechera species at the stages of before and after fertilization. At the premeiotic stage, the expression level of CENH3 in the gynoecium of apomicts was two times lower than that of the sexual Boechera, it decreased in both species by the time of meiosis and increased after fertilization. By 1 DAP CENH3 expression started dropping in sexual B. stricta siliques and kept increasing in apomictic B. divaricarpa ones. That might indicate to a role of CENH3 in apomictic development in Boechera species. The expression levels of APOLLO also sharply decreased by the time of meiosis in gynoecium of both species; however, by 3 DAP, the level of APOLLO expression in siliques of apomicts was almost 1.5 times higher than that of the sexuals. While CENH3 was a single copy gene in all Boechera species, the APOLLO gene have several polymorphic alleles associated with sexual and apomictic reproduction in the Boechera genera. We also discuss polymorphism and phylogeny of the APOLLO and CENH3 genes.
Closely related to the model plant Arabidopsis thaliana, the genus Boechera is known to contain both sexual and apomictic species or accessions. Boechera retrofracta is a diploid sexually reproducing species and is thought to be an ancestral parent species of the apomictic species Boechera divaricarpa. Here we report the de novo assembly of the B. retrofracta genome using short Illumina and Roche reads from 1 paired-end and 3 mate pair libraries. The distribution of 23-mers from the paired end library has indicated a low level of heterozygosity and the presence of detectable duplications and triplications. The genome size was estimated to be equal 227 Mb. N50 of the assembled scaffolds was 2.3 Mb. 27048 protein-coding genes were predicted using a hybrid approach that combines homology-based and de novo methods. Also repeats, tRNA and rRNA genes were annotated. Finally, genes of B. retrofracta and 6 other Brassicaceae species were used for phylogenetic tree reconstruction. Also, a detailed analysis of evolution of the APOLLO apomixis-associated locus was performed. An assembled genome of B. retrofracta will help in the challenging assembly of the highly heterozygous genomes of hybrid apomictic species such as B. divaricarpa.
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