Chromosomal Locations of a Non-LTR Retrotransposon, Menolird18, in Cucumis melo and Cucumis sativus, and Its Implication on Genome Evolution of Cucumis Species

Mobile elements are major regulators of genome evolution
through their effects on genome size and chromosome
structure in higher organisms. Non-long terminal repeat
(non-LTR) retrotransposons, one of the subclasses of trans-
posons, are specifically inserted into repetitive DNA se-
quences. While studies on the insertion of non-LTR ret-
rotransposons into ribosomal RNA genes and other repeti-
tive DNA sequences have been reported in the animal
kingdom, studies in the plant kingdom are limited. Here, us-
ing FISH, we confirmed that Menolird18, a member of LINE
(long interspersed nuclear element) in non-LTR retrotrans-
posons and found in Cucumis melo, was inserted into ITS and
ETS (internal and external transcribed spacers) regions of
18S rDNA in melon and cucumber. Beside the 18S rDNA re-
gions, Menolird18 was also detected in all centromeric re-
gions of melon, while it was located at pericentromeric and
sub-telomeric regions in cucumber. The fact that FISH signals of Menolird18 were found in centromeric and rDNA re-
gions of mitotic chromosomes suggests that Menolird18 is a
rDNA and centromere-specific non-LTR retrotransposon in
melon. Our findings are the first report on a non-LTR ret-
rotransposon that is highly conserved in 2 different plant
species, melon and cucumber. The clear distinction of chro-
mosomal localization of Menolird18 in melon and cucumber
implies that it might have been involved in the evolutionary
processes of the melon (C. melo) and cucumber (C. sativus)
genomes.