Heterochronic Developmental Shifts Underlie Floral Diversity within Jaltomata 1 ( Solanaceae ) 2 3

24 Background: Haeckel and Darwin, respectively, proposed heterochronic shifts during mid to 25 late stages of organismal development as key mechanisms generating phenotypic diversity. To 26 determine whether late heterochronic shifts underlie derived floral morphologies within 27 Jaltomata – a genus with extensive and recently evolved floral diversity – we compared floral 28 development of four diverse species (including an ambiguously ancestral or secondarily derived 29 rotate, two putatively independently evolved campanulate, and a tubular morph) to the ancestral 30 rotate floral form, as well as to an outgroup that shares this ancestral floral morphology. 31 32 Results: We determined that early floral development (<1 mm bud stage) is very similar among 33 all species, but that different mature floral forms are distinguishable by mid-development (3-4 34 mm stages) due to differential growth acceleration of corolla traits. Floral ontogeny among 35 similar mature forms remains comparable until late stages (>5 mm), followed by species-specific 36 heterochronic shifts in corolla traits. 37 38 Conclusions: Our data suggest shared floral patterning during early-stage development, 39 continued shared patterning or re-use of similar floral developmental pathways at mid-stage 40 development, and distinct heterochronic shifts during late-stage development. Heterochrony thus 41 appears to have been important in the rapid and repeated diversification of Jaltomata flowers. 42 43