Preprint: Robertsonian translocations in Danish sika deer may act as barrier to hybridisation with red deer

A bioRxiv preprint reports chromosomal analysis of 56 sika deer (Cervus nippon) and 22 red deer (Cervus elaphus) from Denmark, investigating the basis for hybridisation between the two species — a question carrying regulatory weight following the EU's reclassification of sika deer as an undesirable invasive species.

All red deer in the study had the standard chromosome complement of 2n=68. Sika deer, by contrast, showed chromosome numbers ranging from 64 to 67, attributable to the variable presence of two sika-specific Robertsonian translocations — chromosomal rearrangements in which the long arms of two acrocentric chromosomes fuse, reducing chromosome number. The authors propose that these structural differences may function as markers predicting the absence of fertile first-generation (F1) hybrids between the two species, even where introgression of sika-derived DNA into red deer populations has already occurred from past hybridisation events.

The paper positions these chromosomal variants as potential models for studying selection, speciation, and infertility more broadly. For geneticists and evolutionary biologists, it illustrates how structural chromosomal variation can act as a reproductive barrier even between closely related cervid taxa. For educators and students, the Robertsonian translocation mechanism is a well-established illustration of how chromosomal rearrangements contribute to speciation. This is a preprint and has not yet been peer reviewed.