European Journal of Plant Pathology 103: 113–124, 1997. 113 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Changes in the structure of Arabidopsis thaliana roots induced during development of males of the plant parasitic nematode Heterodera schachtii Miroslaw Sobczak 12 , Wladyslaw Golinowski 1 and Florian M.W. Grundler 2 1 Department of Botany Warsaw Agricultural University (SGGW) ul. Rakowiecka 26/30 02 528 Warsaw, Poland Author for correspondence: 2 Institut f¨ ur Phytopathologie Universit¨ at Kiel D-24098 Kiel, Germany (Fax: +49-(0)431-880 1583) Accepted 19 August 1996 Key words: Heterodera schachtii, nematode, root anatomy, sex determination, syncytium, ultrastructure Abstract Plant parasitic nematodes of the genus Heterodera show a high degree of sexual dimorphism, which is reflected by different nutritional demands and differences in the structure of the induced specific syncytial feeding site in the plant. The determination of the sex of the nematode Heterodera schachtii and other related species was repeatedly reported to be dependent on trophic factors, which are provided by the induced syncytia. The structural differences of syncytia induced by H. schachtii in roots of Arabidopsis thaliana were analysed at the anatomical and ultrastructural level. Syncytia of males were induced in the root pericycle. The developing syncytium then expanded into procambial or cambial cells of the vascular cylinder. Differentiated vascular elements were not included. The expansion of the syncytium triggered the proliferation of cambial and peridermal tissues, in a manner similar to secondary growth, and the formation of additional xylem and phloem elements. In comparison to syncytia associated with females, syncytia associated with males were less hypertrophied and were composed of more cells. Distinct cell wall openings were mostly found between the few strongly hypertrophied syncytial elements at the actual feeding site in the pericycle. The ultrastructure was very similar to female-associated syncytia but showed conspicuous differences in the structure and localization of cell wall ingrowths. These ingrowths were rare and weakly developed and occurred not only at the interface with xylem elements but also at the internal and external walls of the syncytia. After feeding had ceased at the end of the third developmental stage the syncytia degenerated. Abbreviations: J2 – second stage juvenile; J3 – third stage juvenile; J4 – fourth stage juvenile. Introduction Sedentary plant parasitic nematodes have evolved numerous specific adaptations to their habitats (Grundler and Wyss, 1995). One common feature of this group of nematodes is the induction of specif- ic feeding structures in the host plants. An example of the extent to which the parasites have adapted is the mechanism of sex determination in cyst nema- todes. Under favourable environmental conditions the majority of the juveniles develop to females that swell during development and acquire a lemon-like body shape, under adverse conditions the proportion of males increases. In the adult stage males become ver- miform and mobile again. This phenomenon has been explained by two contrary concepts: either a partic- ular sensitivity to environmental influences causes a selective mortality of females (Triantaphyllou, 1973), or the sex of juveniles is determined under the influ- ence of environmental factors (Mugniery and Bossis, 1985; Grundler and Wyss, 1995). A number of studies strongly supports the latter concept of environmentally influenced, epigenetic sex determination. By a varia- tion of nutritional factors or parasitism in plants of different host suitability it was possible to raise the proportion of one sex, whereas the mortality of the whole tested population remained stable (Betka et al.,