Abstract Detail

Phylogenomics

Hodel, Richard [1], Zimmer, Elizabeth [2], Wen, Jun [3].

Wide hybridization followed by aneuploidy explains the origin of the apple tribe.

For nearly a century, plant biologists have debated the origin of the apple tribe (Maleae; formerly Maloideae, Rosaceae). There is considerable variation in chromosome number among major lineages of the Rosaceae, and frequent whole genome duplications in the family, leading researchers to favor allopolyploidy as an explanation for the origin of the Maleae. The ‘wide-hybridization hypothesis’ formulated in the 1930s posits that the Maleae (base chromosome number x = 17) are the result of an ancient hybridization event between progenitors from other tribes in the subfamily Amygdaloideae that have x = 8 and x = 9 respectively. An alternative ‘spiraeoid hypothesis’ proposed that the 17 chromosomes found in Maleae arose via the genome doubling of an x = 9 ancestor to x = 18, and subsequent aneuploidy resulting in x = 17. A genetic investigation using one nuclear gene favored the spiraeoid hypothesis while rejecting the wide-hybridization hypothesis; this study inferred that an ancestor of the tribe Gillenieae (x = 9), which is sister to the Maleae, experienced genome doubling and subsequent aneuploidy. However, the two competing hypotheses have not yet been tested using genomic data. Recent phylogenomic studies identified pervasive cytonuclear conflict throughout the Amygdaloideae, which contains the Maleae, suggesting that ancient hybridization and/or allopolyploidization may have impacted the diversification of this group.

Here, we use publicly available genomic data—448 nuclear genes and complete plastomes— from 27 representative species in the Rosaceae to investigate the origin of the apple tribe. The 27 species represent all three Rosaceae subfamilies and all major tribes within the Amygdaloideae. We use coalescent species tree approaches and analyses of phylogenetic conflict to infer nodes with high levels of gene tree conflict, and network analyses to infer if a reticulate tree explains the data better than a bifurcating topology. Our results suggest that there was a hybridization event between ancestors of tribes Sorbarieae (x = 9) and Spiraeae (x = 9), which gave rise to the clade comprised of Gillenieae + Maloideae (x = 17). Our results support the origin of the common ancestor of the Maleae + Gillenieae via hybridization between distantly related tribes in the Amygdaloideae. However, our results also partially support the spiraeoid hypothesis—the ancestors involved in the allopolyploidization event both had x = 9, so the genome doubling was followed by aneuploidy to result in x = 17 observed in the Maleae.

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1 - National Museum of Natural History
2 - Smithsonian NMNH, Botany MRC 166, P.O. Box 37102, Smithsonian National Museum Of Natural History, Washington, DC, 20013, United States
3 - Botany, MRC-166 National Museum Of Natural History, 10th St. & Constitution Ave., NW, Mrc 166, Washington/DC, 20013, United States

Keywords:
Cytonuclear discord
Ancient hybridization
Reticulate Evolution
Amygdaloideae
Rosaceae.

Presentation Type: Oral Paper
Session: PHYLOV, Phylogenomics V
Location: Virtual/Virtual
Date: Friday, July 23rd, 2021
Time: 3:15 PM(EDT)
Number: PHYLOV002
Abstract ID:875
Candidate for Awards:None