Chromosome number and ploidy-level in a Dutch population ofCrenobia alpina Dana (Planaria)

Abstract

A Dutch population of Crenobia alpina was found to have a chromosome number of 2n = 42. One pair of large chromosomes is acrocentric and probably shows a secondary constriction; one small pair is acrocentric as well. All other chromosomes are metacentric or sub-metacentric; it has not been possible to discriminate with certainty between them. According to the hypothesis of DAH~ (1958) populations with 2n = 42 are autohexaploid, the basic genome consisting of seven chromosomes. On basis of the karyotype it was concluded that the Dutch population described here, is not autohexaploid but functionally diploid, perhaps allohexaploid.

Distribution, ecology, mode of reproduction and karyology of freshwater planarians (Platyhelminthes; Turbellaria; Tricladida) in the springs of the central Pyrenees

“…In external appearance and morphology of the reproductive apparatus, C alpina from the central Pyrenees was basically similar to most monopharyngeal European populations described so far Its basic karyotype differs, however, from those reported by Dahm (1958) in northern Europe He found six mitotic chromosome numbers, 28,35,42,49,56 and 63, which suggested a basic, haploid, complement of seven Diploid and tnploid populations were not found, so, most populations represented four-to nme-ploids A very different sit-uaUon was reported in sexual populations from Holland by Van den Linden (1969) In spermatogonial divisions 42 chromosomes were found, with 21 bivalents in the spermatocytes Hence, these specimens were functional diploids derived from polyploid forms through a process of diploidisation Although meiotic figures could not be examined in our material, the chromosome number and the karyotype derived from mitotic cells are in basic agreement with the Van den Linden results Its polyploid origin can still be traced from the clustering of its chromosomes into seven groups of six chromosomes each However, several pencentric inversions in middle and small-size chromosomes and the presence in the largest set of chromosomes of two large metacentncs together with four smaller chromosomes, two metacentnc and two submetacentnc, makes chromosome matching and true polyploidy untenable and diploidisation from polyploid forms very likely (Ribas 1990). This is a further proof of the selective advantages of polyploid forms (with large cells) in cold habitats On the other hand, the presence of populations of the same species with two different basic chromosome numbers (n = 7 and n = 21) suggests again that, similarly to Phagocata vitta.…”