Cytology and Reproduction of Helicotylenchus Dihystera and H. Erythrinae 1)

Abstract

Several populations of the monosexual species Helscotylenchus dihystera were studied with respect to their chromosomal complement and mode of reproduction to clarify the question of digonic hermaphroditism. No synapsis of homologous chromosomes takes place and maturation consists of a single mitotic division. The somatic chromosome number observed at anaphase or telophase of the maturation division and at prometaphase of the first cleavage division was 2n = 38 in two populations from North Carolina (with possible variation from 38 to 40), 2n = 34 in one population from North Carolina, and 2n = 30 in two populations from Wisconsin. Cleavage proceeded without fertilization. Reproduction in H. dihystera is, therefore, by mitotic parthenogenesis; there is no evidence of hermaphroditism. Cytology and reproduction were also studied in a single population of the bisexual species H. erythrinae from North Carolina. Synapsis of homologous chromosomes takes place, and maturation is normal, consisting of two divisions. The somatic chromosome number observed at prometaphase of oogonial divisions is 2n = 10 and the reduced chromosome number seen at anaphase of the first maturation division and metaphase of the second maturation division of oocytes is n = 5. Re-establishment of the somatic chromosome number takes place through fertilization. No substantial differences in size and shape of the chromosomes of these two species were observed. This is interpreted as indicating the existence of polyploidy in H. dihystera.