Natural pathways towards polyploidy in animals: the Squalius alburnoides fish complex as a model system to study genome size and genome reorganization in polyploids.

When comparing the known picture of polyploidy in animals and in plants, it is possible to recognize some similarities, namely: (i) multiple and recurrent origins in several well-established taxonomic groups; (ii) a strong and regular association with hybridization events; (iii) the production of genotypic diversity; (iv) a rapid genomic reshuffling; (v) a very active role of transposable elements in allopolyploids; (vi) a comparatively privileged occurrence in harsher environments when compared with their diploid relatives, and (vii) gene silencing and divergence of duplicated genes without disruption of duplicated loci. Research on polyploidy was highly biased towards plants during the last century because polyploidy in animals was for long time considered rare, occasional and irrelevant from an evolutionary perspective. However, as empirically observed in plants, genome rediploidization starts in polyploid organisms immediately after the polyploid shock. Given the speed and dynamicity of this process, evidence of genome multiplication is completely erased over time, and hence, only the most recent events are likely to be acknowledged. Although varying in expression between and within taxonomic groups, polyploidy and hybridization are ubiquitous in animals and may be recurrent, fostering evolution. Since evolutionary allopolyploid genomes behave as biologically diploid, zoologists have to challenge the old paradigm of an irrelevant evolutionary role in animals using current genomic and cytogenomic tools. These methods are most likely to reveal the role of polyploid mechanisms in producing evolutionary novelties. Nonsexual complexes are the perfect models to bridge the gap between empirical and theoretical research, while the evolutionary process is in action. Such animal complexes represent a transient stage that, in general, moves towards a polyploid stage, where bisexuality might be recovered, ultimately giving rise to a new gonochoric species. These pathways are herein illustrated by the Iberian allopolyploid Squalius alburnoides. Some general aspects on this fish's complex are updated and reviewed, namely the reproductive modes of the distinct genomotypes, since variable ploidies and genomic combinations occur in natural populations. Most recent data on the mechanisms of gene expression regulation and the importance of the genomic context in driving allelic expression are also included. It was first demonstrated that a regulatory mechanism involving dosage compensation by gene-copy silencing exists in allotriploid females and that allelic expression patterns differed either between genomically equivalent individuals or within the same individual (between tissues and genes). Thus, instead of a whole haplome inactivation, a biased silencing towards repression of a specific allele was observed as well as a reduction of the transcript levels to the diploid state. See also sister article focusing on plants by Tayalé and Parisod in this themed issue.