B chromosomes: a physiological enigma

Abstract

SUMMARYThe concept of the 'nucleotype' embraces aspects of the nuclear DNA that affect the phenotype other than through the coding sequences. In this context, we consider one particular situation, namely DNA variation due to the presence of supernumerary B chromosomes (Bs); these Bs, which are additional to the basic complement of A chromosomes, generate a spectrum of DNA amounts, and thus nucleotypes, within many species. We consider the physiological consequences of carrying extra chromosomes that appear by and large to lack coding sequences, and focus on two species with contrasting case histories: rye (Secale cereale L.) and chives (Allium schoenoprasum L.), which illustrate 'selfish' and 'adaptive' explanations of the persistence of B chromosomes.

Formation and Expression of Pseudogenes on the B Chromosome of Rye

“…These effects are usually cumulative, depending upon the number and not the presence or absence of Bs. In low numbers, Bs have little if any influence on the phenotype, but in high numbers they often have a negative influence on fitness and fertility of the carrier organism (Jones and Rees, 1982;Carlson, 2009;reviewed in Jones, 1995 andJones, 1997). Bs are not necessarily inert, although their moderate effects suggest a lack of major functional genes.…”

The origin and evolution of a unisexual hybrid:Poecilia formosa

“…B-chromosomes have traditionally been classified as selfish genetic elements that decrease the fitness of the 'host' genome (Shaw & Hewitt 1990;Camacho et al 2000); however, there are a few examples where Bs are beneficial to their hosts (Bougourd & Jones 1997). In the Amazon molly, wild animals containing supernumerary B-chromosomes have been found quite frequently in the Río Purificació n/Río Soto la Marina river system (Lamatsch et al 2004) where the species co-occurs with wild-type pigmented P. mexicana and in the laboratory where the fish are bred with black molly, but not in other areas of the range of the Amazon molly (Schlupp 2005).…”

Meiotic and mitotic behaviour of B chromosomes of ryegrass

“…Furthermore, their number varies, within certain limits, amongst individuals of a population, and as such generate a spectrum of DNA amounts, and thus nucleotypes, within the species (BOUGOURD & JONES, 1997). In the present context, in according to JONES et al (2008), we are especially interested in the way in which they contribute to intraspecific variation in nuclear DNA amounts, as well as to the qualities of the additional DNA which they contribute.…”

“…In the last case, Bs induce instability in the As chromosomes (RHOADES et al, 1967;RHOADES & DEMPSEY, 1972;RHOADES & DEMPSEY, 1973;ALFENITO & BIRCHLER, 1990;CHIAVARINO et al, 2000). Such alterations may induce deleterious quantitative phenotypical effects, particularly related to seed fertility reduced, seed germination and flowering time delayed, heading date delayed and seed weight increased (RHOADES et al, 1967;BOUGOURD & JONES, 1997;GONZÁLEZ-SÁNCHEZ et al, 2004).…”