Ancestral state reconstruction reveals extensive homoplasy in nutlet characters of Cynoglossinae (Boraginaceae, subfam. Cynoglossoideae, tribe Cynoglosseae)

Recent phylogenetic studies have challenged the traditional classification of the subtribe Cynoglossinae, which was based on nutlet characteristics. To investigate the evolution of fruit traits related to dispersal modes in this complex group, we conducted a study using scanning electron microscopy to examine 28 taxa representing all previously recognized lineages of the subtribe. Cynoglossinae displays four main types of nutlets: marginate, emarginate, flat wing, and incurved wing. Our findings reveal the arrangement of glochids and their structure, including the number of apical hooks, and their surface ornamentation is highly variable both across and within these four main types. We reconstructed the phylogenetic tree with four main clades (i.e., Clades I-IV) using Maximum likelihood analysis. In addition, we mapped four nutlet traits (i.e., nutlet margin, nutlet glochid, glochid sculpture, and glochid hooks) associated with different dispersal modes using ancestral state reconstruction analysis. The ancestor of subtribe Cynoglossinae was inferred to have marginate and glochidiate nutlets, with glochids externally provided with complex spiny tubercles and bearing at least four hooks at the tip as well as epizoochory dispersal mode. These ancestral states have been retained in Clade II (East Asian-Australian-African Cynoglossum-Lindelofia clade) and Clade III (Mediterranean-Asian Cynoglossum-Solenanthus-Trachelanthus clade), with several independent shifts to emarginate fruits. Transitions from marginate to wide-winged nutlets have occurred at least four independent times, facilitating wind-assisted dispersal in Clade I (Microparacaryum clade), Clade IV (Paracaryum-Mattiastrum-Rindera clade), Rindera tetraspis, and Mattiastrum crista-galli (both belonging to Clade III), where a transition from tuberculate to papillate surface has also occurred.

Automated summary

Recent phylogenetic studies have revealed significant variation in fruit traits within and among different lineages of the subtribe Cynoglossinae. The scanning electron microscopy examination of 28 representative taxa led to the identification of four main types of nutlets and their variations. The reconstructed phylogenetic tree and ancestral state reconstruction analysis shed light on the evolution and dispersal modes of these fruit traits.