Abstract
Apomixis (asexual seed formation) in angiosperms occurs either sporophytically, through adventitious embryony, or gametophytically, where an unreduced female gametophyte (embryo sac) forms and produces an unreduced egg that develops into an embryo parthenogenetically. Multiple types of gametophytic apomixis occur, and these are differentiated based on where and when the unreduced gametophyte forms, a process referred to as apomeiosis. Apomeiotic gametophytes form directly from ameiotic megasporocytes, as in Antennaria-type diplospory, from unreduced spores derived from 1st division meiotic restitutions, as in Taraxacum-type diplospory, or from cells of the ovule wall, as in Hieracium-type apospory. Multiple types of apomeiosis occasionally occur in the same plant, which suggests that the different types occur in response to temporal and/or spatial shifts in termination of sexual processes and onset timing of apomeiosis processes. To better understand the origins and evolutionary implications of apomixis in Boechera (Brassicaceae), we determined apomeiosis type for 64 accessions representing 44 taxonomic units. Plants expressing apospory and diplospory were equally common, and these generally produced reduced and unreduced pollen, respectively. Apospory and diplospory occurred simultaneously in individual plants of seven taxa. In Boechera, apomixis perpetuates otherwise sterile or semisterile interspecific hybrids (allodiploids) through multiple generations. Accordingly, ample time, in these multigenerational clones, is available for rare meioses to produce haploid, intergenomically recombined male and female gametes. The fusion of such gametes could then produce segmentally autoploidized progeny. If sex re-emerges among such progeny, then new and genomically unique sexual species could evolve. Herein, we present evidence that such apomixis-facilitated speciation is occurring in Boechera, and we hypothesize that it might also be occurring in facultatively apomictic allodiploids of other angiospermous taxa.
Highlights
The genus Boechera (Brassicaceae) evolved about 2.5 Myr ago (Mandakova et al, 2015) and is closely related to Arabidopsis (Bailey et al, 2006; Rushworth et al, 2011)
The apomictic hybrids are identified by genome composition as found in the Boechera Microsatellite Website (BMW) http://sites. biology.duke.edu/windhamlab/ (Li et al, 2017). We show that both apospory and diplospory occur frequently in Boechera and are widely dispersed across the genus
Diplospory and Apospory Are Common in Boechera
Summary
The genus Boechera (Brassicaceae) evolved about 2.5 Myr ago (Mandakova et al, 2015) and is closely related to Arabidopsis (Bailey et al, 2006; Rushworth et al, 2011). Boechera includes hundreds of genomically distinct diploid, triploid and tetraploid hybrids that are partially to fully sterile sexually These hybrids produce most of their seeds through apomixis (without meiotic recombination, chromosome reduction or fertilization), but sexually derived seeds are occasionally produced (Aliyu et al, 2010). Another clade of nine taxa, previously assigned to the genus Borodinia (Alexander et al, 2013), is here included in Boechera due to the recent discovery of inter-clade hybridization (Windham et al, field observations) The latter are distinctive in being sparsely pubescent and restricted to forested regions of eastern North America and the Russian Far East. The chromosome base number reduction likely occurred thereafter by multiple translocations (Mandakova et al, 2015)
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