Establishing the cell biology of apomixis in Boechera

The genus Boechera (Brassicaceae) is comprised of both sexual and apomictic species, permitting comparative analyses of meiotic disturbances and parthenogenesis. Hybridization, polyploidy and environmental adaptation that arose during the evolution of Boechera may serve as (epi)genetic regulators of apomictic initiation in this genus.

A conceptual model of events during reproduction of diploid B. stricta.
A conceptual model of events during reproduction of diploid B. stricta. (A) Despite high self-crossing (circular solid arrows), intraspecific crossing within or between populations could happen (circular dashed arrows). While rare, pollination of sexual diploid plants with reduced pollen that introduces apomixis (epi-)genetic factors could contribute to the ubiquity of diploid apomictic inter- or intra-hybrids across the genus that generated variability. Backcrossing with a B. stricta parent makes hybrid-origin plants indistinguishable from pure B. stricta. However, other events, such as (epi-)genetic changes, could bring and/or maintain new (here apomictic) features, without hybridity. (B) Differences occurring during the main stages of sexual development were reminiscent of hybrid or apomictic features. During megasporogenesis there is heterochrony in the ovule (integument) growth inside one ovary, and also in the meiotic events (longevity of the MMC and dyad stages), and the presence of many triads. During gametogenesis there is a lack of female gametophyte development with some ovules and non-functional anthers with degenerated pollen grains. During seed development there is disruption of embryogenesis or embryo abortion, a low number of viable seeds, but in some cases parthenogenetic development. (C) New features of B. strictaindividuals may accumulate (by selfing) and be transferred by intra- or interspecific crossing, leading to the generation of new genotypes. ZLS, zygote-like structure.

Rojek et al. focus on B. stricta, a primarily sexually reproducing species which is also characterized by apomictic individuals, thus implying variation from strictly sexual seed formation. They address this reproductive conundrum through molecular and cyto-embryological investigations of flower development. The results imply that B. stricta is a species with an underlying ability to initiate apomixis. The findings demonstrate that the existence of apomixes in an otherwise diploid sexual B. stricta may provide the genomic building blocks for establishing highly penetrant apomictic diploids and hybrid relatives.

Reference List

Rojek, J., Kapusta, M., Kozieradzka-Kiszkurno, M., Majcher, D., Górniak, M., Sliwinska, E., … Bohdanowicz, J. (2018). Establishing the cell biology of apomictic reproduction in diploid Boechera stricta (Brassicaceae). Annals of Botany, 122(4), 513–539. https://doi.org/10.1093/aob/mcy114