In a number of cockroach species, females are known to switch to an asexual mode of reproduction when isolated from males. The resultant offspring are always females, that is, these cockroaches display facultative thelytokous parthenogenesis. The phenomenon is known in Blatta orientalis, B. germanica, Byr. fumigata, E. lapponicus, E. pallidus, N. cinerea, P. americana, P. fuliginosa, Polyphaga saussurei, and Su. longipalpa (Roth and Willis, 1956; Barth, in Roth and Stay, 1962a; Brown, 1973a; Xian, 1998). Not all females of N. cinerea can reproduce by parthenogenesis; only those with a high level of heterozygosity are capable, and the ability tends to run in families (Corley et al., 2001). Parthenogenesis is rather common in P. americana, and can persist through two generations in the laboratory (Roth and Willis, 1956). Asexual reproduction, however, is clearly a fallback strategy that results in significantly reduced fitness in comparison to mated females. Nauphoeta cinerea virgins produce 10-fold fewer offspring than mated females, and nymphs are less viable, take longer to develop, have shorter adult life spans, and produce fewer offspring of their own when mated (Corley and Moore, 1999). Asex-ually produced oothecae, embryos, and hatched nymphs are often visibly deformed (Griffiths and Tauber, 1942a; Roth and Willis, 1956; Xian, 1998), and in Ectobius, few nymphs develop beyond the second instar (Brown, 1973a). Although the chromosome numbers of asexually produced embryos of N. cinerea ranged from 2n = 19 to 40, only those with the karyotype typical of the species (2n = 36) completed development to the hatching stage (Corley et al., 1999). Extreme variation in embryonic development within an ootheca can cause failure of the entire clutch. If few eggs develop, nymphs may be trapped in the oothecal casing, as hatch seems to require a group effort even in the thin, membranous oothecae of ovovi-viparous cockroaches (Roth, 1974b).

Two cockroach species are known to be exclusively parthenogenetic. The best known is the cosmopolitan Surinam cockroach, Pycnoscelus surinamensis. This taxon is the asexual form of its sibling species Pyc. indicus (Roth, 1967b), and includes at least 21 diploid clones derived independently from sexual females and 11 triploid clones produced by backcrosses between clones and Pyc. indicus. There are more than 10 clones of Pyc. surinamensis in the southeastern United States alone (Roth and Cohen, 1968; Parker et al., 1977; Parker, 2002). In laboratory experiments females of Pyc. surinamensis tended to resist the overtures of male Pyc. indicus, but a few did mate and sperm transfer was successful. In these, the oocytes matured at the same rate as in virgins. Fertility was reduced, however, and all of the resultant offspring were female (Roth and Willis, 1961). In the bisexual Pyc. indicus, the oocytes of virgins develop slightly more slowly than those of mated females, but the proportion of oocytes that mature is the same. The oothecae, however, are almost always dropped without being retracted into the brood sac (Roth and Willis, 1961). Sperm in the spermathecae are re quired for normal oothecal retraction in this species (Stay and Gelperin, 1966), and if the ootheca is not quickly retracted, the enclosed eggs desiccate and die (Roth and Willis, 1955c). The evolution of parthenogenesis in Pyc-noscelus, then, was dependent on overriding this dependence on sperm for oothecal retraction.

The number of eggs produced and matured by the ob-ligately parthenogenetic Pyc. surinamensis is significantly less than that produced by sexual reproduction in its sister species (Roth, 1974b). Nonetheless, Pyc. surinamensis readily becomes established in a new location via a single nymph or adult, and has a widespread distribution (Roth, 1998b). It is found in tropical and subtropical habitats throughout the world, and in protected habitats, particularly greenhouses, in temperate climates (Roth, 1974b, 1998b). Its sexual sibling species Pyc. indicus is native to Indo-Malaysia and adjacent parts of Southeast Asia, and has colonized islands in the Pacific (Hawaii) and Indian (Mauritius) oceans. Both species may be found around human habitations, and both burrow in soil and are poor flyers. The widespread distribution of the asexual form is undoubtedly due to human transport, but the distribution pattern is also typical of geographic parthenogenesis (Niklasson and Parker, 1996), a condition in which a the-lytokous race has a more extensive distribution than its sexual ancestor (Parker, 2002). Pycnoscelus has been used as a model to explore a variety of hypotheses on the subject (Gade and Parker, 1997; Niklasson and Parker, 1994; Parker, 2002; Parker and Niklasson, 1995).

Until recently, Pyc. surinamensis was the only case of obligatory parthenogenesis known in cockroaches. In 2003, a second case was reported in the Mediterranean blattellid species Phyllodromica subaptera by Knebels-berger and Bohn. The distribution of the sexual and asexual forms was studied by analyzing spermathecal contents and the sex of offspring. As in Pycnoscelus, the distribution of Phy. subaptera exhibits a pattern of geographic parthenogenesis: the asexual form is spread over most Mediterranean countries, while the bisexual forms are restricted to the Iberian peninsula. The partheno-genetic and sexual strains of Phy. subaptera cannot be distinguished by external morphology, suggesting that parthenogenesis is a relatively recent acquisition in the taxon.

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