Diploid (2n = 28) to 22-ploid (2n = 308): Functional trait variation and the large-genome constraint in mulberry ( <i>Morus</i> spp.)

Abstract Mulberry ( Morus spp.), a key moriculture crop, serves a dual role: its leaves are indispensable as exclusive fodder for silkworms, while its fruits, particularly in species such as Morus nigra, Morus laevigata and Morus macroura , are valued as functional foods enriched with antioxidants and vitamins. The genus exhibits remarkable natural chromosome-level variation ranging from 14 to 308, which offers a unique system to investigate the impact of genome expansion on plant function. Herein, the influence of different cytotypes on functional traits associated with cell division, cell size and biomass metrics was studied. Principal component analysis revealed that tetraploid (4x), followed by triploid (3x), exhibits the most favourable cytotype, suggesting a vigorous and balanced expression of polyploid advantage. However, downsizing of many key parameters with increasing ploidy levels was also evident, predominantly at 22-ploid (22x). Furthermore, the estimated plasticity index reveals that the lower limit of cell size and rate-limited attributes influenced the loss of growth superiority of the extreme ploidy level; in turn, data imply the large-genome constraint in mulberry, as the consequence of the functioning of a large genome is likely expensive. Therefore, the present study emphasizes that the benchmarking of ploidy levels for the selection of parental material is essential for improving cellular function and developmental efficiency, thereby optimizing leaf yield for the future sustainability of sericulture.