An easier method to identify the individual genomic composition of allopolyploid complexes

Global analysis of the small RNA transcriptome in different ploidies and genomic combinations of a vertebrate complex--the Squalius alburnoides

The Squalius alburnoides complex (Steindachner) is one of the most intricate hybrid polyploid systems known in vertebrates. In this complex, the constant switch of the genome composition in consecutive generations, very frequently involving a change on the ploidy level, promotes repetitive situations of potential genomic shock. Previously in this complex, it was showed that in response to the increase in genome dosage, triploids hybrids could regulate gene expression to a diploid state. In this work we compared the small RNA profiles in the different genomic compositions interacting in the complex in order to explore the miRNA involvement in gene expression regulation of triploids. Using high-throughput arrays and sequencing technologies we were able to verify that diploid and triploid hybrids shared most of their sequences and their miRNA expression profiles were high correlated. However, an overall view indicates an up-regulation of several miRNAs in triploids and a global miRNA expression in triploids higher than the predicted from an additive model. Those results point to a participation of miRNAs in the cellular functional stability needed when the ploidy change.

Ploidy mosaicism and allele-specific gene expression differences in the allopolyploid Squalius alburnoides

Background

Squalius alburnoides is an Iberian cyprinid fish resulting from an interspecific hybridisation between Squalius pyrenaicus females (P genome) and males of an unknown Anaecypris hispanica-like species (A genome). S. alburnoides is an allopolyploid hybridogenetic complex, which makes it a likely candidate for ploidy mosaicism occurrence, and is also an interesting model to address questions about gene expression regulation and genomic interactions. Indeed, it was previously suggested that in S. alburnoides triploids (PAA composition) silencing of one of the three alleles (mainly of the P allele) occurs. However, not a whole haplome is inactivated but a more or less random inactivation of alleles varying between individuals and even between organs of the same fish was seen.

In this work we intended to correlate expression differences between individuals and/or between organs to the occurrence of mosaicism, evaluating if mosaics could explain previous observations and its impact on the assessment of gene expression patterns.

Results

To achieve our goal, we developed flow cytometry and cell sorting protocols for this system generating more homogenous cellular and transcriptional samples. With this set-up we detected 10% ploidy mosaicism within the S. alburnoides complex, and determined the allelic expression profiles of ubiquitously expressed genes (rpl8; gapdh and β-actin) in cells from liver and kidney of mosaic and non-mosaic individuals coming from different rivers over a wide geographic range.

Conclusions

Ploidy mosaicism occurs sporadically within the S. alburnoides complex, but in a frequency significantly higher than reported for other organisms. Moreover, we could exclude the influence of this phenomenon on the detection of variable allelic expression profiles of ubiquitously expressed genes (rpl8; gapdh and β-actin) in cells from liver and kidney of triploid individuals. Finally, we determined that the expression patterns previously detected only in a narrow geographic range is not a local restricted phenomenon but is pervasive in rivers where S. pyrenaicus is sympatric with S. alburnoides.

We discuss mechanisms that could lead to the formation of mosaic S. alburnoides and hypothesise about a relaxation of the mechanisms that impose a tight control over mitosis and ploidy control in mixoploids.

A polymorphic microsatellite from the Squalius alburnoides complex (Osteichthyes, Cyprinidae) cloned by serendipity can be useful in genetic analysis of polyploids

A new microsatellite locus (SAS1) for Squalius alburnoides was obtained through cloning by serendipity. The possible usefulness of this new species-specific microsatellite in genetic studies of this hybrid-species complex, was explored. The polymorphism exhibited by SAS1 microsatellite is an important addition to the set of microsatellites previously used in genetic studies in S. alburnoides complex, that mostly relied in markers described for other species. Moreover, the SAS1 microsatellite could be used to identify the parental genomes of the complex, complementing other methods recently described for the same purpose..