MOLECULAR EVOLUTION OF THE INTERNAL TRANSCRIBED SPACERS (ITS1 AND ITS2) AND PHYLOGENETIC-RELATIONSHIPS AMONG SPECIES OF THE FAMILY CUCURBITACEAE

Phylogenetic relationships of different members of the family Cucurbit aceae were estimated from sequences of the internal transcribed spacer (ITS1 and ITS2) regions of the nuclear ribosomal RNA genes. Twenty-si x species of different genera belonging to different tribes and severa l subtribes were analyzed. The whole ITS regions were amplified by PCR technique and cloned, and three to five different clones of each spec ies were sequenced; for some species PCR products were sequenced direc tly. ITS1 and ITS2 regions are slightly variable in length, with each length appearing genus-specific. A substitution rate of 3.62 x 10(-9) substitutions per site per year was calculated assuming 40 MYA separat ion time. Phylogenetic relationships inferred from ITS sequences of so me species is in agreement with morphological data, but deviations to the taxonomic classification were also observed. A polyphyletic origin of the New World species must be considered. In the genus Cucurbita d ifferent ''types'' of ITS sequences within one species exist, possibly due to the high frequency of introgression during domestication or du e to polyploidization events; in contrast, low intraspecific variabili ty was detectable in the genus Cucumis, indicating different stages of speciation. (C) 1998 Academic Press.