Comparative Genomics of a Species Radiation: Sequencing the Apple Tribe

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PI: Sarah Mathews (Harvard University)Key Collaborator: Erin MacNeal Rehrig (Fitchburg State University)Species radiations are a striking feature of evolution and may be responsible for much of life's diversity. Yet understanding the evolution of species radiations and how genome evolution progresses in this context remains a central problem in biology. Members of the rose family (Rosaceae) include familiar domesticates such as roses, apples, peaches, almonds, strawberries, and cherries. An intriguing and important radiation within Rosaceae produced the apple, pear, and quince genera, along with several valuable ornamentals. These are members of a clade of about 750 woody species distributed in 35 genera with a base chromosome number of x = 17 (tribe Maleae). The sister group of this clade is the genus Gillenia, which has just two species, both herbaceous perennials with a base chromosome number of x = 9. The Maleae are particularly attractive for the study of genome evolution in the context of a species radiation because of their generally small genome sizes and monophyletic roots in a single whole genome duplication (WGD) that postdates their divergence from Gillenia. Early diverging Maleae are similar to Gillenia in having dry fruits and few species. But after the evolution of the pome, the fleshy fruit found in most Maleae and the basis for their economic importance, they diversified into 32 genera, sorting into lineages with distinctive phenotypic, life history, and ecological traits, including lineages in which repeated hybridization and polyploidy events have contributed to their species-richness. This project will test the hypothesis that the infusion of a vast numbers of duplicate genes via WGD provided evolutionary potential for the radiating Maleae. The specific aims are to: (1) generate genome sequence from a small number of diploid genomes (both pre- and post-WGD) and determine the extent to which genic regions can be assembled and the history of duplicated genes reconstructed in selected Maleae genera; and, (2) establish the best sequencing and assembly approaches for the genome scan of a diploid/tetraploid species pair in Maleae. If successful, the project will provide resources for advancing our understanding of the evolution of economically and ecologically important processes such as fruit development, volatile production, changes in climate tolerance, and resistance to pests through the generation of thousands of new markers for breeding programs as well as genome sequences from two closely related species of Malus that differ with respect to ploidy. This project will provide research training for a postdoctoral fellow and a faculty member from Fitchburg State University, a primarily undergraduate institution in Massachusetts. All sequence data will be deposited and accessible through GenBank and the Genome Database for Rosaceae (GDR; www.rosaceae.org).