Gametogenesis and Fertilization in Rhabdias ranae Walton 1929: I. The Parasitic Hermaphrodite

Abstract

The somatic, diploid, and haploid chromosome numbers were determined in the hermaphrodite (24, 12, eggs 6, spermatozoa 5 or 6). Oocytes in the ovary synapsis zone contain 6 tetrads. The tetrads seem to disappear in the growth zone and reappear in the oocyte just prior to fertilization. After sperm penetration, the tetrads move to the periphery and form 2 polocytes by rapid meiotic divisions. Spermatogenesis occurs in the testis zone. Primary spermatocytes show 5 large tetrads and 2 small heterochromosomal dyads which lag on the spindle during division. Each resultant secondary spermatocyte has 1 small and 5 large dyads. In the equational division, one of the lagging heterochromosomes (X) is lost in a small cytophore as cytokinesis occurs. Two spermatids are produced, one with 5 chromosomes and the other with 6 chromosomes. Fertilization occurs in the seminal receptacle and results in either free-living males (2N = 11) or females (2N = 12).