The Boechera Genus as a Resource for Apomixis Research

Abstract

The genera Boechera (A. Löve et D. Löve) and Arabidopsis , the latter containing the model plant Arabidopsis thaliana , belong to the same clade within the Brassicaceae family. Boechera is the only among the more than 370 genera in the Brassicaceae where apomixis is well documented. Apomixis refers to the asexual reproduction through seed, and a better understanding of the underlying mechanisms has great potential for applications in agriculture. The Boechera genus currently includes 110 species (of which 38 are reported to be triploid and thus apomictic), which are distributed mostly in the North America. The apomictic lineages of Boechera occur at both the diploid and triploid level and show signs of a hybridogenic origin, resulting in a modification of their chromosome structure, as reflected by alloploidy, aneuploidy, substitutions of homeologous chromosomes, and the presence of aberrant chromosomes. In this review, we discuss the advantages of the Boechera genus to study apomixis, consider its modes of reproduction as well as the inheritance and possible mechanisms controlling apomixis. We also consider population genetic aspects and a possible role of hybridization at the origin of apomixis in Boechera. The molecular tools available to study Boechera , such as transformation techniques, laser capture microdissection, analysis of transcriptomes etc. are also discussed. We survey available genome assemblies of Boechera spp. and point out the challenges to assemble the highly heterozygous genomes of apomictic species. Due to these challenges, we argue for the application of an alternative reference-free method for the comparative analysis of such genomes, provide an overview of genomic sequencing data in the genus Boechera suitable for such analysis, and provide examples of its application.

Controlling Apomixis: Shared Features and Distinct Characteristics of Gene Regulation

“…Already in 1939 Darlington proposed that apomixis purely represents an escape from sterility that is often caused by polyploidization or hybridization typically associated with apomixis [26]. While polyploidy has long been regarded as a precondition for apomixis, the identification of diploid apomicts, e.g., in the genus Boechera has changed this view [27]. In Boechera, apomixis arose recurrently by hybridization and intra-specific crosses, and also outside Boechera evidence for hybrid origin is given for an increasing number of apomictic taxa [6,28].…”

“…In order to confer successful reproduction, it is essential that all elements of apomixis and associated developmental processes take place in a coordinated manner. To allow this, these regions might include master or key regulatory genes activating a downstream cascade controlling all major aspects of apomixis, or many linked genes encoded on the apomixis linked loci might be required [27]. So far, different genes have been identified to be linked to the apomixis loci in different species (Table 1).…”

Phylogenetic and Expression Analysis of CENH3 and APOLLO Genes in Sexual and Apomictic Boechera Species

“…Moreover, plants from the Boechera genus are close relatives of Arabidopsis thaliana, which is very well studied in terms of molecular genetics and functional gene annotation [29][30][31]. In apomictic Boechera species, the Taraxacum type diplospory with pseudogamous endosperm development that requires fertilization of the central cell is reported [31][32][33][34]. A special attention deserves apomictic accessions of Boechera divaricarpa, which is known as an interspecific hybrid between sexual species B. stricta and B. retrofracta or a closely related species [35].…”

So Closely Related and Yet So Different: Strong Contrasts Between the Evolutionary Histories of Species of the Cardamine pratensis Polyploid Complex in Central Europe

“…To uncover evolutionary processes underlying high diversity in polyploid complexes, we focus on the Cardamine pratensis species complex, which is widespread in Europe and exhibits extensive but poorly resolved variation in Central Europe. It is one of the most complicated polyploid complexes of the family Brassicaceae, comprising sexually reproducing, allogamous perennials capable of vegetative propagation (Lövkvist, 1956) but with no reports of apomixis (which is extremely rare in this family, Brukhin et al, 2019). It includes a number of species and genetic lineages from the diploid to the dodecaploid level, with both aneuploids and dysploids (arisen via chromosomal rearrangements, Mandáková et al, 2013) documented, which makes it an excellent model system for addressing the reticulate evolutionary histories of polyploid complexes (Franzke and Hurka, 2000;Lihová and Marhold, 2006;Marhold et al, 2018 and references therein).…”