Abstract
The current study investigated the impact of methoprene during different phases of oogenesis in the freshwater crab Travancoriana schirnerae Bott, 1969 (Decapoda: Gecarcinucidae), a non-target organism abundant in the wetlands of Wayanad, Kerala, India. Sublethal doses of methoprene impaired ovarian growth as evidenced by reduction in gonadosomatic values, fall in mean oocyte diameter and histopathological changes in all the phases of oogenesis. A significant drop in the proportion of mature oocytes in the experimental crabs were noticed, i.e. methoprene treatment delayed the growth of avitellogenic oocytes to previtellogenic and previtellogenic to early, middle or late vitellogenic stages. Reduction in proliferation of oogonia in the germinal zone and ruptured oolemma in chromatin nucleolus and perinuclear stage oocytes was persistent. Histopathological changes in the primary vitellogenic oocytes include karyoplasmic clumping, perinuclear space atresia, shrinkage and vacuolation of ooplasm, reduction in size of yolk globules and vacuolated globules and degeneration of follicle epithelium. Methoprene negatively affected yolk platelet formation as evidenced by the irregularly fused and distorted yolk platelets of late vitellogenic oocytes. The observed cellular deformities possibly suggest the direct effects of methoprene on the oocytes through general metabolism and growth or through hormones controlling ovarian growth.
Highlights
Knowledge on the endocrine control of the complex larval developmental processes in insects has led to the introduction of insect hormones and their analogs as insecticides, known as insect growth regulators (IGRs)
The current investigation recorded the histopathological changes induced by methoprene treatment during different phases of oogenesis in T. schirnerae
Considerable reduction in the proportion of primary vitellogenic (PV) oocytes was noticed in the experimental crabs, an indication that methoprene treatment slowed down the progress of PN oocytes to PV stage
Summary
Knowledge on the endocrine control of the complex larval developmental processes in insects has led to the introduction of insect hormones and their analogs as insecticides, known as insect growth regulators (IGRs). Changes induced by methoprene on larval survival (Celestial and McKenney, 1994), juvenile development (McKenney and Matthews, 1990), moult frequency, limb regeneration, growth rate (Olmstead and Le Blanc, 2001; Stueckle et al, 2009), adult mortality (Wirth et al, 2001), acute toxicity (Mortimer and Chapman, 1995), bioaccumulation, metabolic and morphologic effects (Walker et al, 2005; 2010) have received much attention. The current study is an attempt to analyze the impact of methoprene application on different phases of oogenesis in an edible freshwater crab Travancoriana schirnerae, a non-target crustacean, commonly found on the embankments of rice fields, banana and areca plantations of Wayanad district, Kerala, India. Since freshwater crustaceans play a vital role in the trophodynamics of wetland/rice field ecosystems, it is important to assess the impact of methoprene on oogenesis of these ecologically fragile invertebrates
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