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, 101 (6), 1618-21

Cytogenetic Evidence for Asexual Evolution of Bdelloid Rotifers

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Cytogenetic Evidence for Asexual Evolution of Bdelloid Rotifers

Jessica L Mark Welch et al. Proc Natl Acad Sci U S A.

Abstract

DNA sequencing has shown individual bdelloid rotifer genomes to contain two or more diverged copies of every gene examined and has revealed no closely similar copies. These and other findings are consistent with long-term asexual evolution of bdelloids. It is not entirely ruled out, however, that bdelloid genomes consist of previously undetected pairs of sequences so similar as to be identical over the regions sequenced, as might result if bdelloids were highly inbred sexual diploids or polyploids. Here, we employ fluorescent in situ hybridization with cosmid probes to determine the copy number and chromosomal distribution of the heat shock gene hsp82 and adjacent sequences in the bdelloid Philodina roseola. We conclude that the four copies identified by sequencing are the only ones present and that each is on a separate chromosome. Bdelloids therefore are not highly homozygous sexually reproducing diploids or polyploids.

Figures

Fig. 1.
Fig. 1.
(A) The 13 chromosomes of P. roseola. (Upper) Embryo nucleus stained with DAPI (4′,6-diamidino-2-phenylindole) and visualized by fluorescence. (Lower) Karyotype. The nuclear DNA content determined by photometry of G1 oocyte nuclei is ≈2.2 pg (12). (B) P. roseola embryo nuclei hybridized at low stringency to a mixture of all four hsp82-containing cosmids labeled with Alexa 568. Red, fluorescent signals; blue, DAPI-labeled DNA; merged, false-color superposition. (Upper) A metaphase nucleus. (Lower) Two interphase nuclei. Each nucleus shows four signals. Near the four prominent signals, fainter signals sometimes appear, often not directly over the chromosome, possibly because of dispersal of the ≈40-kb target DNA. An example is seen in Upper Left. (C) P. roseola embryo nuclei hybridized at high stringency to pairs of differently labeled probes. Red, fluorescent signal from Alexa 568-labeled probe; green, fluorescent signal from Alexa 488-labeled probe; blue, DAPI-labeled DNA; merged, false-color superposition. Autofluorescence from cytoplasm is particularly visible in the green channel. First row, chromosomes hybridized to a mixture of probe 1 green and probe 2 red; second row, interphase nucleus hybridized to a mixture of probe 1 red and probe 2 green; third row, chromosomes hybridized to a mixture of probe 3 red and probe 4 green; fourth row, interphase nucleus hybridized to a mixture of probe 3 green and probe 4 red. Each probe hybridizes preferentially to a single site. (Scale bar, 5 μm.)

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