Abstract
The crucian carp Carassius carassius (Linnaeus, 1758), is native to many European freshwaters. Despite its wide distribution, the crucian carp is declining in both the number and sizes of populations across much of its range. Here we studied 30 individuals of a putative pure population from Helsinki, Finland. Despite clear external morphological features of C. carassius, an individual was of a higher ploidy level than the others. We therefore applied a set of molecular genetic (S7 nuclear and cytochrome b mitochondrial genes) and cytogenetic tools (sequential fluorescent 4’, 6-diamidino-2-phenylindole [DAPI], Chromomycin A3 [CMA3], C-banding and in situ hybridization [FISH] with both 5S and 28S ribosomal DNA probes) to determine its origin. While all examined characteristics of a diploid representative male (CCAHe2Fi) clearly corresponded to those of C. carassius, a triploid individual (CCAHe1Fi) was more complex. Phylogenetic analysis revealed that the nuclear genome of CCAHe1Fi contained three haploid sets: two C. gibelio and one C. carassius. However the mitochondrial DNA was that of C. gibelio, demonstrating its hybrid origin. The FISH revealed three strong (more intensive) 5S rDNA loci, confirming the triploid status, and an additional 24 weak (less intensive) signals were observed in the chromosome complement of CCAHe1Fi. On the other hand, only two strong and 16 weak 5S rDNA signals were visible on the chromosomes of the CCAHe2Fi male. 28S rDNA FISH revealed four strong signals in both CCAHe1Fi and CCAHe2Fi individuals. CMA3 staining revealed four to six CMA3-positive bands of CCAHe1Fi, while that of diploids contained only two to four. The fact that a polyploid hybrid Carassius female with a strong invasive potential may share morphological characters typical for endangered C. carassius highlights a need to combine genetic investigations of Carassius cryptic diversity with conservation measures of C. carassius in Europe.
Highlights
Hybridization in the broad sense is considered as a transmission of alleles among the genomes of related species [1]
Genetic markers unravelling hidden Carassius diversity. Both natural and anthropogenic introduction of invasive species, habitat loss and degradation, predation, draining of wetland pools and genetic contamination are some of the factors that may eradicate populations of native C. carassius [7,10,18,63]
Our genetic analyses contributed to cytotaxonomy of the endangered C. carassius [25,64] and revealed a haploid C. carassius genome in a triploid hybrid combined with a diploid C. gibelio genome
Summary
Hybridization in the broad sense is considered as a transmission of alleles among the genomes of related species [1]. Hybridization has been recognized as a process of considerable importance for species’ evolution [2], as it can often lead to incomplete reproductive isolation. This is especially true for species like fishes that use external fertilization and experience spatial and/or temporal overlap in spawning habitats. Human activities can lead to a reduction in such habitat variation. These anthropogenic alterations often provide favourable conditions that promote hybridization between native taxa [6,7]. Hybridization between native and non-native taxa is recognised as a significant driver of biodiversity loss [11]
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