Systematics Section / ASPT
Negrón-Ortiz, Vivian .
Has speciation in Consolea (Opuntioideae, Cactaceae) occurred through polyploidy?
POLYPLOIDY is an important genetic mechanism that has contributed to the speciation of plants. Cytological studies have shown that polyploid cytotypes are common in the Opuntioideae and Cactoideae subfamilies of Cactaceae. In the Caribbean, the opuntioid Consolea is represented by nine species, all of which are endemic, with very narrow distributions, and includes species classified as rare or threatened. It is sub-dioecious with male, female and, in a few species, hermaphroditic plants. Standard chromosome counting and flow cytometric analyses were used to determine the chromosome number and ploidy level, to characterize the nuclear DNA content, and to determine whether sex chromosomes are present. In addition microcytes were examined to determine a possible mechanism by which polyploidy could occur. Compared to the base number (x=11), the mitotic counts indicated that Consolea is polyploid; no diploids were found. Histograms of intact nuclei confirmed that all species are polyploid with C-DNA value ranging from 6-9 pg. Male and female sexual morphs had similar DNA content, thus there are no sex chromosomes. During inspection of immature anthers, multiple chromatin bridges between meiocytes were observed in prophase I; these allow chromatin transfer between cells. This process, cytomixis, could explain the formation of meiocytes with no chromatin, and the formation of gametes with unbalanced chromosome numbers. The latter could explain the presence of various pollen grain sizes in Consolea anthers. In conclusion, it is plausible that Consolea is of hybrid origin, since many cacti species, particularly polyploids, are of hybrid origin. It is likely that subsequent speciation in this genus involved polyploidization. A mechanism that could have caused polyploidy is cytomixis, resulting in gametes with extremely variable chromosome numbers, influencing reproduction and promoting speciation.
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1 - Miami University, Department of Botany, Oxford, Ohio, 45056, USA
chromosome and flow cytometry.
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 31st, 2006
Time: 11:15 AM