Germinal development in Philophthalmus megalurus (Cort, 1914) (Trematoda: Digenea).

Abstract

The life cycle of Philophthalmus megalurus includes a minimum of five polymorphic generations: the hermaphroditic adult in the conjunctival sacs of birds, a paedogenetic miracidium which develops in the egg, and the sporocyst, mother redia, and daughter redia which parasitize the snail host. Daughter rediae produce a few granddaughter rediae first and then cercariae. The diploid chromosome number is 20 and gametogenesis in the adult is typical with 10 bivalents appearing during the first maturation division of gametocytes. In all generations, an unequal first cleavage produces a smaller propagatory, and a larger somatic cell. Unequal division of the propagatory cell again results in a larger somatic and a smaller daughter propagatory cell from which the sporocyst develops within the miracidium before the egg hatches. In other generations, repeated unequal division of the daughter propagatory cell results in small, dark germinal cells which may multiply by mitosis. They are interpreted as oogonia which transform directly to oocytes in all generations except cercarial embryos. In that process, each undergoes meiotic prophase to diakinesis with the appearance of 10 bivalent chromosomes, then returns to interphase, grows, and becomes the large “germinal cell” typical of sporocysts and rediae of trematodes. Each oocyte cleaves without polar body formation to form an embryo of the next generation. Reproduction in the germinal sacs accordingly is diploid parthenogenesis and polyembryony is not involved. The term “larval trematodes” for generations reproducing in the molluscan host is inaccurate and should be restricted to miracidia and cercariae, the only stages that undergo metamorphosis.