Abstract
Chromosome behaviour during male meioses has been studied in two bisexual amphimictic populations of two tardigrade species, namely Richtersius coronifer and Macrobiotus richtersi (Eutardigrada, Macrobiotidae). Both bisexual populations exhibit a diploid chromosome number 2n=12 and no sex chromosomes were identified. DAPI staining and C-banding data indicate that all chromosomes of the bisexual population of R. coronifer are acrocentric. In both species, at male meiotic prophase, all six bivalent homologous chromosomes are aligned side by side along their length and show no evidence of chiasmata. However, in the oocytes of both species a chiasma is generally present in each bivalent at diplotene stage. Lack of recombination is previously unknown in tardigrades, but is a well known phenomenon in many other metazoans where it is always restricted to the heterogametic sex. In tardigrades there is no evidence of heterochromosomes, but it does not mean that in tardigrades, the heterogametic sex does not exist. The adaptive and evolutionary significance of achiasmatic meiosis is discussed.
Highlights
Chromosome behaviour during male meioses has been studied in two bisexual amphimictic populations of two tardigrade species, namely Richtersius coronifer and Macrobiotus richtersi (Eutardigrada, Macrobiotidae)
The bisexual population of R. coronifer was collected from a moss growing on beech-treetrunks in a locality near Lago di Pratignano (1480 m a. s. l., northern Apennines, Italy), whilst the bisexual population of M. richtersi was collected from hazel leaf litter at Formigine (80 m a. s. l., Modena, Italy)
Richtersius coronifer has a karyotype of 2n= 12 chromosomes which are very similar to each other in shape and size
Summary
Chromosome behaviour during male meioses has been studied in two bisexual amphimictic populations of two tardigrade species, namely Richtersius coronifer and Macrobiotus richtersi (Eutardigrada, Macrobiotidae). Both bisexual populations exhibit a diploid chromosome number 2n =12 and no sex chromosomes were identified. DAPI staining and C-banding data indicate that all chromosomes of the bisexual population of R. coronifer are acrocentric In both species, at male meiotic prophase, all six bivalent homologous chromosomes are aligned side by side along their length and show no evidence of chiasmata. Many species of tardigrades exist as a number of populations which are morphologically very similar to each other but which differ in sex ratio (bisexual or unisexual populations), ploidy (di-, tri-, or tetraploidy) and mode of reproduction (amphimixis or parthenogenesis). Karyotype and chromosome behaviour during male meioses have in this paper been subjected to a deeper analysis, based on observations in two bisexual populations belonging to two different eutardigrade species
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