From Plant Press, Vol. 25, No. 2, April 2022.
In the 1930s, biologists studying chromosomes in the rose family (Rosaceae) noticed that species in the apple tribe (Maleae) had approximately twice as many chromosomes (base chromosome number = 17) as every other species in the rose family (base chromosome number = 7, 8, or 9). Over the ensuing decades, researchers proposed hypotheses of hybridization and/or genome doubling to explain the evolutionary origin of the apple tribe and the unusually high chromosome counts in the apple tribe.
There were two competing hypotheses explaining the evolutionary origin of the apple tribe, but it was difficult to test them rigorously with the data available at the time. For many years, botanists tested these hypotheses using morphological characters, chromosome count data, and more recently, single-locus genetic data. Until a few years ago, researchers did not have access to genome-scale genetic data to test hypotheses of ancient hybridization and/or genome doubling.
An ancient hybridization event gave rise to the apple tribe (Maleae) in the rose family, Rosaceae. Maleae taxa include (from left) sweet crabapple (Malus coronaria), Callery pear (Pyrus calleryana), and Canadian serviceberry (Amelanchier canadensis). (photos by Richie Hodel and Jun Wen)
Smithsonian scientists Richie Hodel, Liz Zimmer, Bin-Bin Liu, and Jun Wen synthesized recently published phylogenomic data from the nuclear and chloroplast genomes of multiple species in the rose family, and applied novel phylogenetic network and multiply-labeled tree analyses to resolve the origin of the apple tribe. They found that an ancient hybridization event occurred between ancestors of the Rosaceae tribes Sorbarieae and Spiraeeae, likely between approximately 65-50 million years ago. Phylogenetic network analyses indicated this hybridization event gave rise to the clade Gillenieae + Maleae. The tribe Gillenieae is a small clade with follicular fruit sister to the larger pome-bearing apple tribe. In contrast to the Maleae, the Gillenieae do not have elevated chromosome counts, indicating that this hybridization event promoted the diversification of the clade Gillenieae + Maleae, but does not explain the high chromosome number in Maleae.
Another key result was that subsequent allopolyploidy (genome doubling involving two distinct species) between ancestors of the apple tribe likely occurred between approximately 50-35 million years ago. Multiply-labeled tree analyses indicated this event probably followed the divergence of the Gillenieae and Maleae lineages. This genome doubling event explains the elevated chromosome numbers observed in the Maleae by researchers over the past century. The research is published in Frontiers in Plant Science (https://doi.org/10.3389/fpls.2021.820997).
Rosaceae tribe, Amygdaleae, which includes (from left) Japanese apricot (Prunus mume) and almond (Prunus dulcis), was once hypothesized to have played a role in a hybridization event leading to the apple tribe, but recent research indicates the participants in the ancient hybridization event were likely ancestors of the tribes Spiraeeae and Sorbarieae. (photos by Richie Hodel and Jun Wen)