The effects of Secale cereale B-chromosomes have been studied both in some B-chromosome carrying cultivated rye varieties and in various hybrids, primarily with Triticum aestivum.
Rather similar patterns of B-chromosome effects and possible interactions with A-chromosome genes have been described in Lolium (Evans & Macefield, 1973; Taylor & Evan, 1977) and Agropyron (Chen et al.
This means of assessing genome homology, of course, suffers from all the same flaws as traditional genome analysis, but it is additionally based on dubious assumptions about B-chromosome behaviour and effects.
Among the major points of criticism are 1) the lack of relationship between the concept of homology in genome analysis and the "classical" concept of homology in morphology and molecular systematics; 2) the possible influence on chromosome pairing of specific genes, B-chromosomes, the environment, and so forth; 3) the magnitude and pattern of variation of chromosome pairing in hybrids; and 4) the problem of identifying pairing chromosomes during meiosis.
hybrids, B-chromosomes reduce the number of multivalents, restricting chromosome pairing to form bivalents (Evan & Macefield, 1973; Taylor & Evans, 1977).
A general review of B-chromosomes can be found in Jones and Rees (1982) and the effects of B-chromosomes upon the behaviour of the A-chromosome complement during meiosis in grass species and hybrids has been considered previously by Jones (1991).
It has been suggested that B-chromosomes might provide a useful tool for genome analysis in the Triticum/Aegilops complex (Sano & Tanaka, 1983; Ohta, 1991).
A cytological explanation of the effects of B-chromosomes could be that the addition of chromatin to the nucleus prolongs prophase and, hence, prolongs the time available for recombination (Loidl, 1994).
Although the actual mechanisms are unknown through which the B-chromosomes interact with the A-chromosome complement, they cannot possibly change the homology or similarity (structurally or in base composition) of the A-chromosomes.
Apart from B-chromosomes, both euchromatic and heterochromatic supernumerary chromosome segments have been shown to affect chromosome pairing especially in various insect species (e.
with the diploid species of congeneric Aegilops-Triticum complex, based on the new method of genome analysis using its B-chromosomes.