AUTHOR=Mandáková Terezie , Hloušková Petra , Windham Michael D. , Mitchell-Olds Thomas , Ashby Kaylynn , Price Bo , Carman John , Lysak Martin A. 

TITLE=Chromosomal Evolution and Apomixis in the Cruciferous Tribe Boechereae

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.00514

DOI=10.3389/fpls.2020.00514

ISSN=1664-462X

ABSTRACT=<p>The mustard family (Brassicaceae) comprises several dozen monophyletic clades usually ranked as tribes. The tribe Boechereae plays a prominent role in plant research due to the incidence of apomixis and its close relationship to <italic>Arabidopsis</italic>. This tribe, largely confined to western North America, harbors nine genera and c. 130 species, with &gt;90% of species belonging to the genus <italic>Boechera</italic>. Hundreds of apomictic diploid and triploid <italic>Boechera</italic> hybrids have spurred interest in this genus, but the remaining Boechereae genomes remain virtually unstudied. Here we report on comparative genome structure of six genera (<italic>Borodinia</italic>, <italic>Cusickiella</italic>, <italic>Phoenicaulis</italic>, <italic>Polyctenium</italic>, <italic>Nevada</italic>, and <italic>Sandbergia</italic>) and three <italic>Boechera</italic> species as revealed by comparative chromosome painting (CCP). All analyzed taxa shared the same seven-chromosome genome structure. Comparisons with the sister Halimolobeae tribe (<italic>n</italic> = 8) showed that the ancestral Boechereae genome (<italic>n</italic> = 7) was derived from an older <italic>n</italic> = 8 genome by descending dysploidy followed by the divergence of extant Boechereae taxa. As tribal divergence post-dated the origin of four tribe-specific chromosomes, it is proposed that these chromosomal rearrangements were a key evolutionary innovation underlaying the origin and diversification of the Boechereae in North America. Although most Boechereae genera exhibit genomic conservatism, intra-tribal cladogenesis has occasionally been accompanied by chromosomal rearrangements (particularly inversions). Recently, apomixis was reported in the Boechereae genera <italic>Borodinia</italic> and <italic>Phoenicaulis</italic>. Here, we report sexual reproduction in diploid <italic>Nevada</italic>, diploid <italic>Sandbergia</italic>, and tetraploid <italic>Cusickiella</italic> and aposporous apomixis in tetraploids of <italic>Polyctenium</italic> and <italic>Sandbergia</italic>. In sum, apomixis is now known to occur in five of the nine Boechereae genera.</p>