Integrative approach to the systematics of the endemic Svalbard aphid specis Macrosiphum calvulum (Hemiptera, Aphididae) using molecular morphological and reproductive system analysis.

Aphids in the Svalbard archipelago are limited to a few highly specialized species adapted to extreme Arctic conditions. Among them, the endemic species historically identified as Sitobion (Metobion) calvulum remains poorly studied. Its systematic placement has been uncertain due to the lack of fresh material, and key aspects of its reproductive biology and endosymbionts remain unknown. Here, using an integrative approach combining molecular phylogenetics, morphology, and reproductive system analysis, we clarify its taxonomy and biology. Phylogenetic analyses based on mitochondrial COI and nuclear EF-1α sequences reveal its close relationship to Nearctic Macrosiphum species, leading to the establishment of the new taxon combination Macrosiphum calvulum comb. nov. Simultaneously, morphological observations uncover several atypical traits that challenge the established boundaries within Macrosiphini. Ultrastructural studies highlight unique reproductive adaptations, including secretion patterns in male accessory glands and oviparous female spermathecae. The absence of known facultative endosymbionts aligns M. calvulum with other aphids in Svalbard. We used SEM to detail the morphology of the sexual generation and applied TEM and, for the first time in aphids, micro-CT imaging to analyze their reproductive system. Given that Svalbard is among the most climate-threatened regions globally, studying M. calvulum is essential for understanding and conserving Arctic biodiversity.