Mite Chromosomes : an Exceptionally Small Number

Abstract

ALTHOUGH the chromosomes of thousands of plants and animals have been investigated and chromosome numbers found to vary widely, the overall trend has been toward the evolution of a moderate number in most taxa. Very few species have been reported to have as few as n = 2 chromosomes. Jackson¹ reported the flowering plant Haplopappus gracilis to be n = 2 and most cytology textbooks indicate that the nematode Parascaris equorum (= Ascaris megalocephala) possesses one pair of chromosomes in germ line cells. In the latter species, however, the two chromosomes fragment into numerous small chromosomes in the soma, and therefore the germ line chromosomes are considered aggregate bodies². In the Insecta some coccid members of the tribe Iceryini have a haploid number of two chromosomes and unfertilized haploid eggs develop parthenogenically into males³. Sokolov⁴ found two species of water mites (Eylais rimosa and Eylais setosa) to be n = 2, but these species do not display haploid parthenogenesis. The rarity of reported cases of such a low chromosome number throughout the plant and animal kingdoms prompted us to report the following data.