Abstract Detail

Reproductive Processes

Sequence variation in histidine-domain arabinogalactan proteins (HD-AGPs) generates interspecifc prezygotic mating barriers among the core eudicots.

Histidine-domain arabinogalactan proteins (HD-AGPs) are expressed in the transmitting tissue of the pistil. TTS proteins from Nicotiana and AGP31 from Arabidopsis are the best-studied members of this gene family, and TTS proteins have been shown to be necessary for optimal growth of pollen tubes in vivo. Here, angiosperm genomes were surveyed to identify HD-AGP orthologs, and to test the hypothesis that sequence variation in a hypervariable domain (HV2) correlates with post-pollination pre-zygotic reproductive barriers that manifest as failure of pollen tubes to thrive in a conspecific pistil, a phenomenon known as incongruity. HD-AGP orthologs were found throughout the core eudicots, including basal lineages such as Nelumbo, Nyssa, and Vitis. They are absent in the Ranunculids, Magnolids, and monocots. HD-AGPs appear to have evolved from the AGP30 family of arabinogalactan proteins, which lack the histidine domain, are expressed in root tips, and are found in all angiosperms. Among the core eudicots, variation in the number and location of XKPP or VHPP tetrapeptide repeat motifs correlates with the presence of incongruity between pairs of species within a genus. The HV2 region is shorter and the number of tetrapeptide motifs lower in the more basal groups, increasing with evolutionary distance to the most derived members within the asterid lineage and the rosid lineage. Similarity in HV2 motifs predicted absence of incongruity, and ease of interspecific hybridization, even in taxa isolated for millions of years. For example, Populus trichocarpa (section Tacamahaca) from western North America hybridizes with P.alba (Section Leuce) from Europe and North Afica, and as predicted by the hypothesis, the HV2 region is essentially identical between the two species. However, these taxa do not interbreed with P. euphratica (section Turanga), and bioinformatics analysis reveals that two of the tetrapeptide motifs are divergent between the two former species and P. euphratica, which is native to North Africa, the Middle East, and Asia. The same correlation between incongruity and HV2 motif variation were observed in over 40 species-pairs for which hybridization data are available along with complete HD-AGP sequences. These include species of Quercus, Prunus, and Gossypium various cucurbits; and members of the Fabaceae and Solanaceae. The findings support a paradigm that posits rapid reinforcement of reproductive isolation between incipient species via incongruity generated through selection for functional variants of a critical reproductive protein, the HD-AGP.

1 - Western Washington University, Biology, BI 315, Bellingham, WA, 98225, United States

Keywords:
gene flow barriers
reproductive isolation
pistil proteins
arabinogalactan proteins
gene family evolution
Pollen performance.

Presentation Type: Oral Paper
Session: REP3, Reproductive Processes 3
Location: Virtual/Virtual
Date: Thursday, July 30th, 2020
Time: 10:45 AM
Number: REP3004
Abstract ID:682
Candidate for Awards:None