The Chromosomes of Moniliformis dubius (Acanthocephala)

Squashes of testes and ovarian masses of Moniliformis dubius contained some gonial cells with mitotic metaphase configurations which are described and figured. The diploid chromosome number is eight in females and seven in males. An XO sex-determining mechanism is proposed, with the male heterogametic. During the first meiotic division of spermatocytes, the single X chromosome begins polar migration before the autosomes move away from the equator of the spindle. Meiosis in oocytes takes place after sperm penetration. The two X chromosomes move to opposite poles of the spindle before separation of the autosomes in Meiosis I. The differences in chromosome number and behavior between M. dubius and Macracanthorhynchus hirudinaceus are noted. Apart from the studies of Jones and Ward (1950) and Robinson (1964) on the chromosomes of Macracanthorhynchus hirudinaceus, the author is not aware of any published work on karyotype analysis of archiacanthocephalans. In M. hirudinaceus there is a diploid chromosome number of six and an XY sex-determining mechanism, the male being heterogametic. Another archiacanthocephalan, Moniliformis dubius Meyer, 1933, is currently being studied in this laboratory. We are particularly interested in the cytology and development of the parasite together with the nature and extent of cytological and morphological changes which follow X-irradiation. One aspect of the project has been the determination of the chromosome number and the morphology and behavior of the chromosomes during mitosis and meiosis. This was done in order to provide a description of the karyotype of this species which could be used as a basis for comparison of the chromosome complements and sex-determining mechanisms in Mon. dubius and Macr. hirudinaceus. Also, a knowledge of normal chromosome morphology and behavior is necessary before the effects of X-irradiation on the chromosomes can be satisfactorily described. MATERIALS AND METHODS Specimens of Periplaneta americana infected with cystacanths of M. dubius, and laboratory rats infected with adults were kindly provided early in 1963 by Dr. W. L. Nicholas, Zoology Department, The Australian National University, Canberra. From these initial infections, the parasite has been maintained in cockroaches and rats in this laboratory. More recently, cockroaches natReceived for publication 21 November 1964. u ally infected with cystacanths were found in the Sydney area, and from this material additional laboratory infections have been established. Male and female white Wistar rats were killed about 21 days after infection with 10 to 15 cystacanths. Infection rates varied considerably, but seldom fell below 50%. After removal from the host, the worms were washed in physiological saline and squashes of testes or ovarian masses were prepared in the way described earlier (Robinson, 1964). All preparations were examined and all photographs were taken with a Zeiss photomicroscope. Adox KB14 film and Adox E10 film developer were used.