Quantifying niche overlap and transgression in allopolyploid hybrids: Case study of Sorbus subgenus Aria

Apomixis, the formation of seeds without recombination, facilitates adaptation and persistence under environmental change. By preserving hybrid genotypes over long time periods, apomixis may conserve adaptive trait combinations from parental niche margins. We tested whether apomictic entities occupy intermediate, marginal, or transgressive niche space relative to their parents and whether differentiation is associated with ploidy.

We studied polyploid Sorbus subgenus Aria in the Franconian Jura (Germany), comprising two progenitors Sorbus aria and S. collina, seven triploid entities, and a pool of genetically heterogenous individuals (single genotypes). Genetic structure was assessed using MIG-seq. Overall niche differentiation between parental taxa and hybrids was evaluated using Sørensen similarity of two-dimensional hypervolumes derived from principal component analysis (PCA) axes. Niche shifts were further analyzed using hypervolumes based on the three strongest PCA variables. Across 762 occurrences, observations ranged from 11 to 453 individuals per entity.

Environmental niche space was transgressive in three, significantly allocated towards the margins of parental niche space in one, while remaining intermediate in the other entities. Niche transgression occurred towards milder temperatures and drier conditions. Genetic analyses confirmed morphologically defined entities, although one morphotype was polyphyletic. Tetraploid S. collina significantly occupied warmer and wetter environments compared to other cytotypes. Triploids differed from S. aria along microtopographic gradients represented by the second PCA axis.

Apomictic Sorbus entities show diverse strategies in niche occupation and can occupy environmental niche space at and beyond the limits of their parental taxa. Apomicts may conserve evolutionary adaptations at the edges of parental niche space that may otherwise be lost from, or fail to emerge in, the parental gene pool. Over long timescales these trait combinations may re-enter the parental gene pool through introgression, thereby reintroducing adaptations critical for survival under changing conditions.