Interactions in Interspecific Crosses between Members of the Tomato Clade

One premise of the biological species concept (BSC) is that reproductive barriers act to prevent interbreeding between species. While the BSC is not universally applicable, interspecifi c reproductive barriers (IRBs) between species can be detected in many cases. For example, in the tomato clade ( Solanum sect. Lycopersicon ) prezygotic IRBs can prevent hybridization between certain species ( Rick, 1956 , 1979 ; Martin, 1961a , b , 1964 ; Hardon, 1967 ; Rick et al., 1976 ; Liedl et al., 1996 ; Bedinger et al., 2011 ). In compatible crosses, a pollen grain on a stigma germinates, forming a pollen tube that grows through the style and into the ovary to fertilize the ovum ( Cheung, 1996 ; Dresselhaus and Franklin-Tong, 2013 ). However, in incompatible crosses, pollen tubes can be prevented from reaching the ovary by active rejection processes. In a number of wild tomato species, there are two types of pollen–pistil incompatibility systems involving rejection of pollen tubes in pistils. First, self-incompatibility (SI) can prevent inbreeding through the rejection of self-pollen tubes. In the Solanaceae, SI depends on the interaction of S -locus pistilexpressed S-RNases and pollen-expressed F-box proteins, as well as nonS -locus factors, such as pistil HT proteins and pollen SCF ubiquitin ligase components including Cullin1 (CUL1) ( Kao and Tsukamoto, 2004 ; McClure, 2004 ; McClure and Franklin-Tong, 2006 ; Kubo et al., 2010 ; Li and Chetelat, 2014 ). Second, in some crosses between species, unilateral incompatibility (UI) occurs, such that pollinations are compatible in one direction and incompatible in the other direction ( Levin, 1971 ; Grant, 1981 ; Hogenboom, 1984 ; McClure et al., 2000 , 2011 ; Hancock et al., 2003 ). UI barriers, thus, contribute to the reproductive isolation of species. The directionality of UI often 1 Manuscript received 14 December 2014; revision accepted 18 December 2014. The authors thank the Charles M. Rick Tomato Genetics Resource Center for seeds; A. Ashford for plant care; A. Martin, O. Todd, B. Genuchi, and N. Allen for help with compositing micrographs; Dr. S. Royer for help preparing fi gures; and Drs. A. Tovar-Méndez, A. Broz, and A. Randle for critical reading of the manuscript. This work was supported by grant number DBI-0605200 and MCB-1127059 from the Plant Genome Research Program of the National Science Foundation. 5 Author for correspondence (e-mail: [email protected])