What is "cytodeme"

n. A group of organisms that have the same number of chromosomes, and that can form fertile hybrids with each other

Cytodeme first emerged as a printed word more than 60 years ago in a book by Heslop-Harrison. Discussing the Deme Terminology - he continued "cytodeme, a population differing in some distinctive cytological feature from others." More precisely the cytodeme may be defined as the total assembly of all those individuals/organisms that use an identical specific suite of chromosomes to carry their genes. In most cases the suite is composed of several pairs of homologous chromosomes with or without a pair of sex chromosomes. Since the only acceptable proof of the identity ( homology) of chromosomes lies in their ability to pair fully from end to end during meiosis it follows that:

1. In asexual taxa membership of a cytodeme can only be presumed on such evidence as visual similarity of chromosomal ideotype, but never proved.
2. In sexual taxa all of the members of one cytodeme are fully intercompatible (within the limits of sex and other genetical constraints like self-incompatibility mechanisms) yielding the fertile progeny which proves their membership.

As a general rule for most species all of its members are of the same cytodeme excepting only the infrequent aneuploid aberrants. However some species are known to include several chromosome races which must necessarily belong to different cytodemes. Although it is not intrinsic in the formal definition it is a matter of commom observation that members of different cytodemes are essentially cross-incompatible and any hybrids that do arise are usually highly infertile.