Nematodes exhibit a variety of sexual and asexual reproductive methods (reviewed by Evans 1998) and, of the plant-parasitic nematodes, most information is available on Meloidogyne (reviewed by Chitwood and Perry 2009). Sexual reproduction (amphimixis) can occur in species where there are two separate sexes, where the haploid (n) male (spermatocytes) and female (oocytes) gametes fuse to form the zygote and restore the diploid (2n) complement of chromosomes. In A. tritici and D. dipsaci, for example, female nematodes are the homogametic sex (genetically XX) and males are heterogametic (sometimes XO, usually XY), and the sex ratio is determined genetically. In genera such as Globodera, Heterodera and Meloidogyne sex chromosomes are absent and the sex ratio may be environmentally influenced. In many genera, including Heterodera, Pratylenchus and Radopholus, the female can be fertilised by several males, thus enhancing the genetic diversity of the offspring.
In species of nematodes that reproduce asexually, males are absent or occur only rarely. There are two main types of parthenogenesis, meiotic parthenogenesis (au-tomixis) and mitotic parthenogenesis (apomixis). In meiotic parthenogenesis, there is a first meiotic division in the oocytes, although there are variations between species. This meiotic division allows some genetic reorganisation, even though the diploid chromosome number is restored by self-fertilization. Meloidogyne hapla race A exhibits facultative meiotic parthenogenesis, oogenesis and spermatogenesis proceeding as in amphimictic species to yield one haploid nucleus and two polar bodies per oocyte. Although parthogenesis predominates, amphimixis can occur if sperm are present. However, if the sperm are not present, the egg pronucleus recombines with the second (haploid) polar body to restore the diploid state. In monosexual populations of Aphelenchus avenae, females produce only female progeny and reproduction is by obligate meiotic parthenogenesis. Meiosis produces only one polar body, with the egg nucleus having the 2n chromosome number that then develops into the zygote.
Mitotic parthenogenesis, which occurs in several species of Meloidogyne, is the most common method of asexual reproduction and is always obligate; the only division is mitosis and the oocytes retain the diploid chromosome number. This would seem to prevent any genetic reorganisation, except for that resulting from mutations. Frequently, mitotic parthenogenesis is associated with polyploidy, which may increase the likelihood of mutation. Several of the most widespread and economically important species of Meloidogyne are obligate mitotic parthenogens. Populations of the same Meloidogyne species may differ in mode of reproduction; for example, 29 of 32 populations of M. hapla reproduced by facultative meiotic parthenogenesis, the others by mitotic parthenogenesis (Triantaphyllou 1966).
In hermaphrodites, both egg and sperm are produced in the same individual. Usually the sperm is produced first and is stored in the spermatheca, then the go-nad produces oocytes, which are fertilized by the sperm until the sperm supply is exhausted. Hermaphroditism is a common method of reproduction amongst free-living nematodes but is relatively rare in plant-parasitic nematodes, being found in some members of the Criconematoidea and in Radopholus similis and species of Paratrichodorus.
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