An old puzzle revisited:
molecular cytogenetics of sex determination in non-marine ostracods
Radka Symonova1, Renate Matzke-Karasz2 & Stefan Müller1
1
Department of Biology II, Anthropology and Human Genetics, Ludwig-Maximilians-University Munich
82152 Planegg-Martinsried, Germany
2
Department of Environmental and Geosciences, Palaeontology Section,
Ludwig-Maximilians-University, and GeoBioCenterLMU, 80333 Munich, Germany
Introduction: Non-marine ostracods have a wide range of reproductive modes, from fully
asexual, over mixed reproduction to fully sexual. Although most studies on chromosome
morphology and chromosomal sex determination systems of non-marine ostracods were
carried out in the early 1930s and 40s (e.g. Bauer 1934, 1940, Dietz 1954, 1955), they
nevertheless still provide some, and for most species the only available data. The common
pattern appears to be male heterogamy, with either XO- or XY-derived systems. However,
apparently the X chromosome/autosome ratio and not the absolute number of X chromosomes
determine the gender (Dietz 1955). The presence of one or more Y chromosomes does not
necessarily imply that this chromosome is 'the trigger' for male development. The most recent
chromosomal studies on non-marine ostracods by Tétart (1967, 1978, 1983) focusing mainly
on female karyotypes from parthenogenetic populations, showed an evolution towards
multiple sex chromosomes.
Within the framework of the EU Marie-Curie RTN SEXASEX, dealing with the mechanisms
that determine gender and reproductive modes in a species with mixed reproduction, we
employ state-of-the-art molecular cytogenetic techniques for a refined analysis of
chromosomal sex determination systems in non-marine ostracods.
Materials & Methods: Classical staining techniques for invertebrate chromosome provided
only very limited information about the presence, number and structure of sex chromosomes.
The introduction of fluorescence in situ hybridization (FISH) in the late 1980ies, however,
marked a paradigm shift from the analysis of chromosome morphology towards chromosomal
DNA content. We employ a variety of FISH based techniques for Ostracod sex chromsome
identification and analysis, comprised of a) differential fluorochrome staining and replication
banding for chromosome identification, c) comparative genomic hybridisation for the
detection of male/female differences in the sex chromosome/ autosome ratio and d)
chromosomal micro-dissection for the establishment of sex chromosome specific FISH
probes. Ostracod species used in our studies: Heterocypris incongruens and Eucypris virens.
Results: We will provide an overview of the molecular cytogenetic strategy for the
identification and characterisation of Ostracod sex chromosome systems. We will further
present initial results of the karyological studies carried out in Heterocypris incongruens and
Eucypris virens.
Keywords: karyology, chromosomes, sex-determination, fluorescence in situ hybridisation
(FISH)
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