Loss of Anisocotyly in European Gesneriaceae and Its Evolutionary Consequence
Date Issued
2009
Date
2009
Author(s)
Huang, Bing-Hong
Abstract
Anisocotyly is common in Gesneriaceae, which refers to that both cotyledon are unequally developed. The macrocotyledon can continuously grow while microcotyledon ceases to grow after cotyledon expansion. Anisocotyly might be able to help seedlings to adapt/survive in dense shade environments. Unlike other dicot plants, Old World species evolve anisocotylous trait during germination while New World species remain isocotyly instead. Within Old World European Gesneriaceae species (Ramonda myconi and Haberlea rhodopensis), there appears to be a secondary loss of anisocotylous trait back to isocotyly. These isocotylous species thus provide a great opportunity to investigate how anisocotyly develop, and loses during Gesneriaceae evolution. In this study, I compared cotyledon development in isocotylous European Gesneriaceae species to Streptocarpus rexii, their sister clade species with true anisocotyly. The results of cotyledon size measurement showed that European Gesneriaceae species exhibit nearly equal cell growth rate and thus maintain both cotyledons in the same size. But in Streptocarpus rexii, growth rate between macrocotyledon and microcotyledon was different thus altered cotyledon allometry growth. Further examinations of cell size, trichome distribution and meristematic activity failed to find any differences between cotyledons of European species. All observations showed that both cotyledons of European Gesneriaceae species ceased to grow in early stages and thus no prolonged meristematic activity could be observed. Previous studies indicated that meristematic activity gene such as Class 1 KNOX (STM, BP) may play a role in the formation of anisocotyly. I therefore isolated KNOX gene homologues from several Gesneriaceae species in the hope to find the correlation of transition of isocotyly and anisocotyly to any corresponding KNOX homologues evolution. The results so far did not indicate any correlation perhaps due to the lack of detailed samplings. Further expression and functional studies on these genes will be helpful to understand their roles on anisocotyly. Nonethesless, this study indicated that European Gesneriaceae species is isocotylous because of no differential cell division, cell expansion, trichome distribution and lateral vein formation between both cotyledons. The species phylogeny indicates European isocotylous species are perhaps a loss of anisocotyly.
Subjects
Anisocotyly
Gesneriaceae
Meristematic activity
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