Embryology of the astigmatid mite Scancassania berlesei

Research project P 14691 Embryology of the astigmatid mite Sancassania berlesei Manfred Günther WALZL 09.10.2000 The mites are the most species-rich taxon of the arachnids. They play an important role both from the biological- ecological as well as the medical and economic perspective. Nonetheless, our knowledge about their embryonic development remains fragmentary. Earlier embryological investigations have concentrated largely on the Ixodida and Mesostigmata. The proposed project focuses on the embryogenesis of the less well-studied mite order Astigmata. The current project is designed to contribute to a better understanding of the organization (bauplan) of the Astigmata embryos. On the one hand, this will close considerable gaps in our knowledge, on the other hand it will enable a comparison between the organization of Astigmata embryos with those of other mites and arthropods in general. Finally, it will allow us to better understand the systematic position of the Astigmata within the mites. Particular emphasis will be placed on the mode of cleavage, endoderm development, the Jate" of the fourth leg pair, and problems pertaining to germ band development. The literature available on this topic contains methodological uncertainties stemming from the applied light microscopic procedures and the insufficient number of developmental stages studied. In order to avoid these uncertainties, the embryonic development of Sancassania berlesei MICHAEL, 1903 - the animal investigated here will be examined in full and without interruption from the fertilized egg cell to the hatching stage of the larva using transmission and scanning electron microscope techniques.

The Acari (mites) are the most species-rich group of arachnids and play an important role both from the medical- health and economic perspective. Nonetheless, their embryogenesis is difficult to study and poorly known, primarily due to the impermeability of the eggshell for fixing liquids. Past embryological studies have concentrated mostly on larger ticks and predatory mites, revealing a considerable embryogenetic plasticity. The smaller storage mites, whose population explosions can cause severe economic damage, have only been investigated with light- optical techniques. In order to clarify the often contradictory results obtained in the past, we therefore examined the easily reared astigmatic storage mite Sancassania berlesei using the electron microscope. Early development, which takes place in the mother, was clarified in detail. A special technique was developed for the scanning electron microscope and transmission electron microscope; it enabled the impermeable eggshell to be dissolved, exposing the ca. 80 m long embryos for subsequent electron microscopical study. Treating the eggs in the household cleaning and bleaching liquid Danchlor (=sodium hyperchloride solution) and applying a newly developed diamond knife yielded ultra- and semithin serial sections; these were then photographed digitally and reconstructed 3- dimensionally with the aid of a computer. The following results were obtained: Fertilization takes place in the ovary of the mother, whereas early development takes place in the directly adjoining oviduct, which is subdivided into 3 distinct parts. Meiosis and the accompanying constriction of the polar bodies takes place in the 1st part of the oviduct. In the 2nd part of the oviduct, nutritive substances are deposited in the egg as yolk bodies via the oviduct wall. In the 3rd and longest part of the oviduct, embryonic development starts with synchronous holoblastic cleavage via a 2-, 4-, 8-, and 16-cell stage. After the first three cleavage divisions, cleavage becomes asynchronous and the yolk material is unevenly distributed among the daughter cells. The interior is characterized by a few large, yolk-rich cells that are surrounded by a cell layer consisting of small, rounded, yolk-free cells. After the egg had been deposited by the female, the small yolk-free cells give rise - via cell flattening and polarization - to the closed cell aggregation of the blastoderm. The development of this new treatment technique for embryos therefore enabled us to clarify the disputed type of cleavage: holoblastic cleavage has thus been proved for the astigmatic mites.