In the animal kingdom, two subtypes of true parthenogenesis have been described, depending on the presence of chromosome pairing. In both cases, the oocyte nucleus undergoes a single maturation division and generally emits a single polar body. In the first subtype, the chromosomes are paired during oogenesis, and recombination can occur between them. In the second subtype, pairing does not occur, and the single maturation division is equational, but apparently without recombination. Both meiotic and mitotic parthenogeneses have been described in plant parasitic nematodes, such as Meloidogyne spp. (Triantaphyllou and Hirschmann, 1964). Karl Bělař (see Section 18.3) described the cytology of meiosis in free-living nematodes with true parthenogenesis, using a single, undescribed, species, Rhabditis XIX (Figure 7). This species displayed diakinesis and pairing, followed by separation of sister chromatids after anaphase. Here, in contrast to usual female meiosis, focused centrosomes remained during meiosis (thus alleviating the need for a spermatozoon to provide them) (Bělař, 1923; Bělař, 1924).
Even true parthenogenetic free-living nematodes have been little studied. True parthenogens are found in genera in the sister clade to Caenorhabditis, such as Diploscapter (Brauchle et al., 2009), the Panagrolaimus, Halicephalobus, Rhabditophanes and Cephalobus/Acrobeloides genera (Cephalobina suborder), and aquatic species such as Plectus sp. The species Diploscapter coronatus (Rhabditidae) contains a single chromosome pair, which undergoes a single meiotic division without pairing (although the polar body later divided; Hechler, 1968; Lahl et al., 2006). In Acrobeloides nanus (Cephalobidae), a second meiotic division was observed but the two daughter cells then fused. Plectus sp. ES601 (Plectidae) produces two polar bodies; whether diploidy is achieved in this species through an additional round of DNA replication is unclear (Lahl et al., 2006). A better understanding of genetic segregation in parthenogens has been only achieved in some plant parasitic nematodes, such as Meloidogyne hapla (Liu et al., 2007).
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