The Chromosomes of Hymenolepis microstoma (Dujardin 1845)

Abstract

The chromosomes of Hymenolepis microstoma, studied in Feulgen-stained squash preparations of germ cells and early embryos, are 12 diploid. Their average size in first cleavage metaphase is 2 to 5 p. The inequality in size of some homologous chromosomes is explained in terms of origin from unequally contracted pronuclear chromosomes. Centromeres are terminal; heterochromatic regions exist near them. Meiosis in the oocytes and mitosis and cleavage in the early embryos are briefly described. Papers by Jones et al. (1945, 1951, 1956, 1957), Kisner (1961), and Douglas (1962) on the chromosomes of various cestodes have created interest in further cytological investigation of this group. Hymenolepis microstoma, the bile duct tapeworm of mice, became available in 1959 for laboratory use. The present study is part of a series of investigations into the biology of this worm (Dvorak et al., 1961). MATERIALS AND METHODS For the analysis of chromosomes, 20 normal worms belonging to a strain which has been maintained in our laboratory for the past 3 years were removed from two mice 14 days after infection and fixed in Carnoy's fixative (6:3:1 absolute alcoholchloroform-glacial acetic) for at least 12 hr. The specimens were then hydrated, hydrolyzed in water bath at 60 C in 1 N HCI for 12 min, rinsed in cold 1 N HC1, and stained in Feulgen's leukobasic fuchsin (prepared according to Gray, 1954) for at least 1 hr. The stained materials were preserved in distilled water in the refrigerator. Squash preparations were made of a small region of the anterior third of each worm using a drop of 45% acetic acid and sealing the cover glass with clear fingernail polish. The chromosomes of selected cells were drawn with the aid of an oil immersion objective (N.A. 1.40) and a camera lucida. Chromosomes from about 50 slides of normal oocytes and early embryos were studied. Since chromosomal details were observed best in oocytes and early embryos, the present study was largely confined to these stages. RESULTS Normal chromosome complements in somatic cells of the early embryo typically consist of 6 pairs, or 12 diploid. Of these, four pairs of homologues are of small size (2 to 3 4), one pair is of intermediate size (4 p), and one pair is definitely larger than the rest (5 4). The average size range, therefore, in first cleavage metaphase is 2 to 5 p. In some cells the two members of the largest pair are somewhat unequal in size (Fig. 10); in others, this difference in size cannot be clearly observed. The location of the centromere is not clearly visible in the metaphase chromosomes, but from the observations f early anaphase of first cleavage it is obvious t at all centromeres are terminal or very nearly so. Separation in early anaphase always begins at one end of the metaphase chromosomes. Also, in anaphase all chromosomes are rod-shaped (Fig. 8) as expected if centromeres are terminal. In some metaphase figures of first cleavage, there appear some lightly stained regions near one end of the chromosomes. These regions resemble subterminal centromeres, a suggestion which is not supported by studies of anaphase figures. The lightly stained portions should be considered heterochromatic regions extending from the terminal centromeres. Because the chromosomes of the testes are quite small, little was learned from testis material other than confirmation of the haploid number from meiotic metaphase in primary spermatocytes (Fig. 1). As the ovaries mature, they become lobed and packed with germ cells. The germ cells Received for publication 16 August 1962. * This study was supported in part by U. S. AEC contract AT(40-1) 1749. t On deputation from the Pakistan Atomic Energy Commission in a doctoral program sponsored by the U. S. Agency for International Development.