DNA transposon expansion is associated with genome size increase in mudminnows

Genome sizes of eukaryotic organisms vary substantially, with whole genome duplications (WGD) and transposable element expansion acting as main drivers for rapid genome size increase. The two North American mudminnows, Umbra limi and U. pygmaea, feature genomes about twice the size of their sister lineage Esocidae (e.g., pikes and pickerels). However, it is unknown whether all Umbra species share this genome expansion and which causal mechanisms drive this expansion. Using flow cytometry, we find that the genome of the European mudminnow is expanded similarly to both North American species, ranging between 4.5-5.4 pg per diploid nucleus. Observed blocks of interstitially located telomeric repeats in Umbra limi suggest frequent Robertsonian rearrangements in its history. Comparative analyses of transcriptome and genome assemblies show that the genome expansion in Umbra is driven by extensive DNA transposon expansion without WGD. Furthermore, we find a substantial ongoing expansion of repeat sequences in the Alaska blackfish Dallia pectoralis, the closest relative to the family Umbridae, which might mark the beginning of a similar genome expansion. Our study suggests that the genome expansion in mudminnows, driven mainly by transposon expansion, but not WGD, occurred before the separation into the American and European lineage. Significance StatementNorth American mudminnows feature genomes about twice the size of their sister lineage Esocidae (e.g., pikes and pickerels). However, neither the mechanism underlaying this genome expansion, nor whether this feature is shared amongst all mudminnows is currently known. Using cytogenetic analyses, we find that the genome of the European mudminnow also expanded and that extensive chromosome fusion events have occurred in some Umbra species. Furthermore, comparative genomics based on de-novo assembled transcriptomes and genome assemblies, which have recently become available, indicates that DNA transposon activity is responsible for this expansion.