The polyploid paradox in mulberry: enlarged genome exhibits loss of growth superiority

Mulberry (Morus spp.), one of the key sericultural crops, exhibits remarkable cytogenetic diversity, with natural chromosome-level variation ranging from 2n=14 to 2n=308. This broad spectrum—from diploid, triploid, tetraploid and hexaploid to the extreme polyploid decosaploid—offers a unique system to investigate the impact of genome duplication on plant physiology and morphology. We studied the influence of different cytotypes on traits associated with cell division, cell size, and biomass metrics. Principal Component Analysis (PCA) revealed that tetraploids exhibit the most favorable combination of traits, suggesting a vigorous and balanced expression of polyploid advantage. Beyond tetraploidy, many key parameters showed signs of downsizing with increasing ploidy levels, particularly in hexaploid and decosaploid genotypes. Moreover, improved storage capacity coincided with significantly higher LMA and a greater number per stomata. Collectively, inconsistent growth superiority reflects higher ploidy levels, suggesting that phenotypic expression in polyploids is influenced by the lower limit of cell size (stomatal length, stomatal width, and guard cell volume) and rate-limited attributes (leaf length, leaf width, leaf area, and petiole length). These patterns imply a trade-off: while moderate polyploidy (tetraploid) enhances morphological robustness, excessive polyploidy may incur growth penalties due to biophysical and nutrient constraints. Therefore, the current study highlights that benchmarking genome size/ploidy level for parental material selection is a crucial factor that may enhance cellular function and developmental efficiency, thereby maximizing leaf yield. Considering the inverse relationship between ploidy and growth performance, as well as biophysical and nutrient constraints, necessitates a deeper understanding of molecular mechanisms.