Morphological Comparison of Members of the Heterodera Trifolii Species Complex 1)

Abstract

A detailed morphological and morphometric comparison was made by light and scanning electron microscopy of six populations of the Heterodera trifolii species complex (including H. galeopsidis and H. lespedezae) with chromosome numbers of 2n = 24, 26, 27, 28, 32, and 34. Among nineteen morphometric characters studied in second-stage juveniles, twelve characters (body length, stylet length, stylet knob height, stylet knob width, dorsal esophageal gland orifice distance, tail length, tail terminus length, stylet knob width/height, tail length/tail terminus length, tail terminus length/ stylet length, tail length/body width at anus, caudal ratio A) were useful in differentiating the populations. Certain qualitative characters such as shape of stylet shaft and knobs, shape and rate of tapering of tail and size of tail annulation were also helpful. Cluster and discriminant analyses, based on seven morphometric characters, were only partially successful in identifying juveniles as to the populations they belonged to. In females, stylet length, and in cysts, various structures of the cyst cone, were the best distinguishing characters. Of fourteen morphometric characters of cyst cones, only five (fenestra length, dorsal semifenestra width, fenestra length/width, vulval slit length, vulva-anus distance) were helpful. The scanning electron microscope revealed additional external structural differences in cyst cones of the various populations in the structure of vulval area of unfenestrated and fenestrated cysts in top view including number and structure of radial ridges; number, shape and size of cuticular ridges between radial ridges and anus (perineal ridges) in side view; and structure of anal area. Limited host tests on eight plants showed that Trifolium repens, T. pratense, Lespedeza striata and Vicia villosa were the most favorable hosts for all six populations, whereas Rumex crispus, Dianthus caryophyllus and Pisum sativum were less favorable. None of the populations reproduced on Rumex acetosella. On the basis of these studies, it is concluded that the chromosomal forms of the H. trifolii species complex could be recognized as taxonomic units, preferably as subspecies, to emphasize their close phylogenetic relationship.