The possibility of occurrence of this type of regulation brings
up several questions not only regarding the functionality of
gene dosage-compensation systems but also within the
context of polyploid species evolution.
Here we report a gene-regulation mechanism involving
gene-copy silencing in triploid S. alburnoides. Whether this process might be exclusive to this complex is still to be determined,
and divergent clues come from distinct groups. In the
allotriploids of the genus Poeciliopsis, triallelic expression in
a number of allozyme loci and the assessment of ploidy on
the basis of allelic dosage suggest that all genome copies
are contributing to overall expression and that no silencing occurs
at the loci under analysis [16, 17]. Conversely, in other
studies involving the triploid unisexual Poecilia formosa, indistinguishable
expression between diploids and triploids was
observed at several allozyme loci [18]. More recently, individual
variation in pigmentation phenotypes resulting from
crosses involving Poecilia formosa individuals indicates differential
contribution of genomes to overall expression and suggests
that a comparable gene-copy silencing process may
also occur in this polyploid species [19]. Evidence of a mechanism
that leads to a diploid state of genic activity also comes
from comparisons between protein contents and enzyme activities
of tetraploid and diploid counterparts of teleost
[20, 21] and anuran [22] species. Thus, the same mechanism
could also have a role in the regulation of gene expression in
autopolyploids, but the possibility of a widespread occurrence
among the different groups is yet to be demonstrated. Autopolyploid
systems, however, offer less experimental potential
in terms of understanding the gene-regulation process
because, unlike allopolyploids such as S. alburnoides or
P. formosa, the contribution of individual genomes cannot
be identified on the basis of genome-specific sequence polymorphism.