Abstract
BackgroundSqualius 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.ResultsTo 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.ConclusionsPloidy 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.
Highlights
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 described as an allopolyploid hybridogenetic complex, where allopolyploid refers to an increased ploidy level and hybrid genome composition of particular forms within the system; hybridogenetic refers to an alternative mode of reproduction; and complex is the technical terminus denoting a natural system composed of parental species and their hybrids, with altered modes of reproduction and reproductive interdependence [10]
We could exclude the influence of this phenomenon on the detection of variable allelic expression profiles of ubiquitously expressed genes in cells from liver and kidney of triploid individuals
Summary
(a) Intra-organ differences in ploidy - Detection of mosaic individuals A total of 40 fish were analysed using flow cytometry for ploidy determination in blood, liver and kidney cell suspensions: four S. pyrenaicus, three nuclear non-hybrid S. alburnoides and 33 hybrids S. alburnoides (Table 1). B1) Gene expression patterns according to organ and geographical location The allele expression pattern of ß-actin, rpl and gapdh genes of BK and BL cells was assessed for a total of 20 individuals pooled from the Tejo, Almargem and Guadiana samples (Table 3). The expression pattern of triploid PPA’s from Tejo was determined for ß-actin, rpl and gapdh genes, and it was found to be biallelic for the 3 genes in both organs The genotype of both A and B cells in liver and kidney of controls for expression mosaic (non ploidy mosaic triploid Sq18, Sq29 and Sq31) was PAA, and the expression outcome was biallelic (Table 3) for all the.
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