Degenerate tetraploidy was established before bdelloid rotifer families diverged

Rotifers of Class Bdelloidea are abundant freshwater invertebrates known for their remarkable ability to survive desiccation and their lack of males and meiosis. Sequencing and annotation of approximately 50-kb regions containing the four hsp82 heat shock genes of the bdelloid Philodina roseola, each located on a separate chromosome, have suggested that its genome is that of a degenerate tetraploid. In order to determine whether a similar structure exists in a bdelloid distantly related to P. roseola and if degenerate tetraploidy was established before the two species separated, we sequenced regions containing the hsp82 genes of a bdelloid belonging to a different family, Adineta vaga, and the histone gene clusters of P. roseola and A. vaga. Our findings are entirely consistent with degenerate tetraploidy and show that it was established before the two bdelloid families diverged and therefore probably before the bdelloid radiation.     

Orientation of repeating units in Xenopus chromosomal ribosomal DNA: A test of a stochastic model for maintaining intraspecies homogeneity

The orientation of repeating units in chromosomal ribosomal DNA of Xenopus was determined by denaturation mapping in order to test a model for gene rectification. The model postulates an alternating orientation that will allow the formation of pallindromic loops between adjacent repeats. Mutations would then be lost or driven to fixation by random correction of one sequence by another, following genetic homogeneity among the repeats. Head-to-tail orientation was found, ruling out the model for this system.