Double spermatogenesis in Chelicerata

Abstract

Sperm dimorphism is a rare phenomenon in Chelicerata. Until now, it was known only from three species of the opilionid genus Siro (Sironidae, Cyphophthalmi). Fertilizing (eusperm) and nonfertilizing spermatozoa (parasperm) develop in the same cyst and are thus sister cells. The fine structure of the spermatozoa of two species has been examined and is compared here. In contrast to Siro rubens, S. duricorius spermatozoa lack an acrosomal complex. Both sperm types produce a transitional process, a more or less modified flagellum, which is later retracted. Hence, the spermatozoa are aflagellate. Eusperm and parasperm of all three species form highly ordered sperm balls that are stored in the deferent duct. Reviewing and adding new results about the sperm dimorphism in this arachnid taxon provides the basis for some considerations of another enigmatic morphological character found in Uropygi and Amblypygi, i.e., the tubular accessory genital glands that show holocrine extrusion. These glands are suggested to represent modified, infertile derivatives of the testis anlage. Their secretion is produced in a way reminiscent of a strongly degenerated spermatogenesis. Consequently, these products may be regarded as strongly degenerated germ cells representing a line of germ cell development, which has been separated very early in spermatogenesis from the usual line leading to fertilizing sperm cells. This further, although less evident, case of probable dichotomous germ cell development is discussed with respect to the controversial phylogenetic-systematic relationships between Uropygi (Thelyphonida and Schizomida), Amblypygi, and Araneae.

Gross morphology, histology, and ultrastructure of the alimentary system of Ricinulei (Arachnida) with emphasis on functional and phylogenetic implications

“…7A), it is regarded to be “postcerebral.” Postcerebral sucking pumps are known as “sucking stomachs” from Araneae and Amblypygi (e.g., Snodgrass,1948; Foelix,1996; Weygoldt,2000). The presence of a postcerebral sucking stomach has been quoted as strong synapomorphy supporting the sistergroup‐relationship of Amblypygi and Araneae (“Labellata” of Weygoldt and Paulus,1979a,b) in contrast to the Pedipalpi‐hypothesis (see also Alberti,2005). The presence of this posterior pump in Ricinulei seems to weaken this character and thus to weaken the Labellata‐hypothesis.…”

Evolutionary and biogeographical history of an ancient and global group of arachnids (Arachnida: Opiliones: Cyphophthalmi) with a new taxonomic arrangement

“…All 19 multistate characters were unordered. Spermatogenesis in Cyphophthalmi is a promising source of phylogenetic characters, as recently outlined by Alberti, Giribet & Gutjahr (2009; see also Juberthie & Manier, 1976; Juberthie, Manier & Boissin, 1976; Juberthie & Manier, 1978; Alberti, 1995, 2005), although taxon sampling is still sparse and these characters were not considered in this data set (G. Alberti & G. Giribet, unpubl. data).…”

A phylogenetic analysis of the arachnid orders based on morphological characters

“…Recent work indicates that Amblypygi, or whipspiders, is the sister group to Uropygi s.l. (Shear et al ., 1987; Shultz, 1990, 1999; Giribet et al ., 2002; but see Alberti, 2005) and together form the clade Pedipalpi. Earlier studies tended to favour a sister‐group relationship between Amblypygi and Araneae, the spiders (e.g.…”