Discussion

At first glance, when not in flower, the new species P. exserta appears to belong to sect. Aleuritia because of its dwarf habit, the presence of farina on the leaves and calyx, and its leaf morphology. However, the distinctive characteristics of its floral morphology—the 7–8-lobed corolla and calyx, corresponding 7–8 stamens, and filaments and style extending beyond the petals—might even lead one to suspect that it represents a new genus. Nevertheless, upon closer examination, it is evident that the filaments are inserted at the middle of the corolla tube, are adnate to the tube, and become free and extend beyond the petals only at the mouth of the tube; the style is also elongated and protrudes well beyond the corolla tube. Based on these floral traits, it is hypothesized that this species may have evolved from a long-styled (pin) flower in which both the filaments and style elongated in response to environmental pressures, thereby facilitating adaptation to a specialized habitat. The precise mechanisms underlying these morphological changes require further confirmation through molecular biological studies. This new species is distributed in southeastern Yunnan Province, a region characterized by a relatively hot climate. In contrast, most species of sect. Aleuritia occur in temperate and alpine regions. Among the currently known members of this section, P. exserta may represent the southernmost species in the Northern Hemisphere. It is therefore plausible that the specialized floral morphology of this species is a consequence of adaptive evolution following speciation. Instances in which particular floral traits in Primulaceae have deviated from the typical Primula morphology and led to recognition at the generic level have occurred previously. For example, the former sect. Auganthus has been treated as a distinct genus on three separate occasions, with its distinctive calyx morphology resulting in its recognition under three heterotypic names: Auganthus Link, Oscaria Lilja, and Primulidium Spach (Richards 2002). Furthermore, as noted in previous studies, Primula luquanensis Z.K.Wu & Wei Zhou and Primula nutantiflora Hemsl., both possessing the fifth type of floral morphology within the genus Primula, are currently placed in sect. Aleuritia (Wu et al. 2023a). Therefore, based on habit, farinose condition, and leaf morphology, this new species is assigned to sect. Aleuritia.

The leaf shape of P. exserta resembles that of P. membranifolia, but its nearly scapeless habit and distinctive floral structure readily separate it from the latter. In terms of leaf morphology, the absence of a scape, and the presence of farina on both the leaf blades and flowers, P. exserta is similar to P. pengzhouensis and P. mianyangensis. However, it differs from both in having a 7–8-lobed corolla and calyx, corresponding 7–8 stamens, and elongated filaments and a style exserted far beyond the mouth of the corolla tube.

The discovery of this new species enriches the diversity of the genus Primula and also provides valuable material for future studies on floral evolution within the genus.