Chromosome Morphology and Behavior in Macracanthorhynchus hirudinaceus

Abstract

Gonads from Macracanthorhynchus hirudinaceus females and males were stained with aceto-orcein, then squashed and examined for morphological changes and behavior of chromosomes during mitosis and meiosis. Mitotic configurations occur in the female in the oogonia of the ovarian masses. The diploid, metaphase karyotype is composed of one homologous pair of acrocentric chromosomes and two metacentric pairs. In males at mitotic metaphase in spermatogonia, there is a pair of acrocentric chromosomes, a metacentric pair, and a third pair consisting of one metacentric and one subacrocentric chromosome, the latter being regarded as the Y chromosome. In the spermatocytes at late metaphase of Meiosis I, the sex chromosomes separate from the autosomal mass and soon afterwards migrate to opposite poles of the spindle before terminalization of chiasmata in the two autosomal bivalents. Few investigations have been carried out on the chromosomes of the Acanthocephala. This is somewhat surprising in view of the continued interest in other aspects of the biology of acanthocephalans and also in view of the considerable interest recently in the chromosomes of parasitic worms of other groups, notably the cestodes and trematodes. Walton (1959) lists chromosome numbers for five species in the entire phylum. Of the palaeacanthocephalans listed, Pomphorhynchus laevis (= Pomphorhynchus proteus) apparently has a diploid number of eight (Von Voss, 1910) while Echinorhynchus gadi (= Echinorhynchus acus), Acanthocephalus ranae (= Echinorhynchus haeruca), and Polymorphus minutus (= Echinorhynchus polymorphus) have 16 chromosomes in the diploid set (Hamann, 1891). Hamann's records represent some of the earliest published counts of parasite chromosomes. The fifth recorded chromosome number is that of the archiacanthocephalan Macracanthorhynchus hirudinaceus (Pallas, 1781). Until Jones and Ward (1950) established the diploid number in this species as being regularly six, counts varying between four and seven had been given (Meyer, 1928). In addition to citing the chromosome number, Jones and Ward (1950) described briefly the morphology of the chromosomes and proposed an XY sexdetermining mechanism in which the male is heterogametic. The present study was undertaken, primarily, to describe and figure morphological changes and movement of chromosomes during cell division in a readily obtainable acanthocephalan, as the first step in a cytogenetic study of the group. MATERIALS AND METHODS Living adults of M. hirudinaceus are found attached to the wall of the small intestine of the pig. Worms were removed soon after slaughter of the hosts at the Homebush abattoirs in Sydney, Australia. The pigs came from various areas within the state of New South Wales. Worms were placed in insulated flasks containing 0.85% NaCl at 38 C and brought to the laboratory as soon as possible. Chromosome preparations were usually made within 6 hr of collection of worms, but on one occasion mitotic stages were observed in the gonads of worms which had been maintained in 0.85% NaCl at 38 C for 52 hr. Somatic nuclei in acanthocephalans are thought to cease mitotic division early in embryogeny. Accordingly, in adults, oogonia and spermatogonia were examined for stages of mitosis. Meiosis in females occurs preceding release of primary oocytes from ovarian masses; in fact, it occurs after sperm penetration of the oocyte and after the appearance of the membranes which subsequently surround the developing embryo. Since adult females containing oocytes at this stage were not found, meiosis was studied in males only. In female worms, the gonads consist of ovarian masses which are present in large numbers in the Received for publication 4 May 1964.