Willowsia jacobsoni

Systematics of Willowsia jacobsoni: molecular and developmental insights and the challenges of revealing synapomorphies for the main groups of Willowsia According to Zhang et al. (2011), in a phylogenetic hypothesis based on 54 morphological characters, W. jacobsoni could be related to W. mexicana and Americabrya arida, but this clade was not well supported, in addition to some mistakes in the interpretation of a few characters regarding W. jacobsoni (e.g. characters 12, 21, 22, 36, 37). Still, W. mexicana needs to be investigated using molecular markers, since it shares several similarities with W. jacobsoni, such as Ant IV with unilobed apical bulb; labral papillae with projections; macrochaetotaxy dorsal pattern of the head (except by M1, M4i, Pa2, Pa4, Pp3), including the cervical transverse row of mac; reduction of mac on Th II–III; and Abd II–IV formula with 2+1, 2+3, 4(5)+12(15) mac (Zhang et al. 2007). From a molecular perspective, neither W. jacobsoni nor W. mexicana has yet been analyzed using a robust set of markers (eg. UCE, USCO). Comparing the 10 species of Willowsia used in our study, there is no strong morphological evidence that supports the obtained groups, except the absence of p1 mac on Th III which is exclusive to W. guangdongensis and W. pseudobuski, and the presence of m5 mac on Th II only seen in W. fascia, W. qui, W. pseudobuski and W. similis (Table 3). Another possible synapomorphy is a5 mac on Th III shared between the six Chinese species of Willowsia, but it is also present in W. buskii. Even though it is not exclusive, and the three Willowsia species (W. buski, W. nigromaculata, W. neonigromaculata) within the W. jacobsoni clade lack some mac on Th II (m1, m2, m5) and Th. III (a2, a3, m5i) (Zhang et al. 2011; Cipola & Katz 2021). The shared features in these clusters may not be synapomorphic traits, as certain characteristics can be present in phylogenetically distinct groups (e.g. labral papilla with projection in W. jacobsoni and W. pyrrhopygia) (Fig. 14). The lack of morphological characteristics able to support the split of polyphyletic Entomobryinae genera like Entomobrya and Willowsia (Zhang et al. 2014, 2015, 2016, 2017, 2019; Katz et al. 2015b; Ding et al. 2019; Cipola & Katz 2021) makes difficult further advances in the subfamily supra-specific systematics. In addition, the low representation of many taxa from these groups in phylogenetic analyzes can generate uncertain groupings, especially for species-rich genera that occur in poorly surveyed biogeographic areas. Thus, this study not only provides valuable data to the understanding of W. jacobsoni, but also underscores the importance of integrating molecular and morphological approaches in addressing the evolutionary history of entomobryid springtails.