Chromosomes of Heterobilharzia americana (Digenea: Schistosomatidae) from Texas

Abstract

Heterobilharzia americana is 1 of 2 known species of mammalian schistosomes indigenous to North America and is the only species in its genus. Reported vertebrate hosts include dogs, raccoons, nutria, and other wild animals (Goff and Ronald, 1981, American Journal of Veterinary Research 42: 1775-1777). Snail hosts are Fossaria cubensis and Pseudosuccinea columella (Malek, 1967, Journal of Parasitology 53: 700702). Various biological studies have been reported by Malek et al. (1961, Journal of Parasitology 47: 619-623), Lee (1962, Journal of Parasitology 48: 728-739), and others. Grossman et al. (1981, Chromosoma 84: 413-430) briefly described chromosomes of H. americana. They reported a diploid number of 20, with a heteromorphic pair of sex chromosomes in the female (ZW) and a homomorphic pair in the male (ZZ). The present paper contains a more detailed description of the chromosomes of the same strain of Heterobilharzia americana from Texas. The stock of Heterobilharzia was supplied by Dr. Norman C. Ronald, who sent snails (Fossaria cubensis) that had been exposed in his laboratory to multiple miracidia. The miracidia originated from eggs in feces of a naturally infected raccoon that had been collected near College Station, Texas. The snails had been collected from the same area before exposure in the laboratory. Chromosomes studied were from material in sporocysts from 8 snails. The sex of schistosome infection in 5 snails was verified by infection of mice with cercariae and recovery of adult worms. Chromosome preparations were made by airdried methods published earlier (Short and Grossman, 1981, Journal of Parasitology 67:661671). Results are based on observations on over 200 metaphase and prometaphase plates. Measurements (Table I) were made on photographs by means of dividers from 10 male and 10 female plates. The relative length of a chromosome is the quotient of its individual length divided by the length of the haploid complement (which was calculated thus: (2A + 2Z)/2). Each centromeric index was calculated by division of 100 x the length of the short arm by the total individual chromosome length. Table I presents information on relative chromosome lengths, centromeric indexes, and classification following the terminology of Levan et al. (1964, Hereditas 52: 201-220). In favorable preparations, all chromosomes could be identified (Fig. 1). The distal end of the short arm of chromosome 5 is characteristically attenuated, producing a lightly stained area where the nucleolar organizer region appears to be. Occasionally what appears to be a small satellite was seen distal to the lighter area on 5 (Fig. 1 D). On the plates measured (Table I), the centromeric position sometimes was not clear on chromo-