The 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.