Relationships of Myrtopsis, Correa and Leionema

We provide high support for a clade of Correa, Leionema and Myrtopsis, three genera that previous studies have been unable to confidently place (Bayly et al. 2013; Appelhans et al. 2021); this group was recovered as an unsupported clade by Duretto et al. (2023). This clade contains probably the most varied morphological characters in the Eriostemon group. The recovery of a sister relationship between Leionema and Correa is congruent with the plastid sequence phylogeny of Duretto et al. (2023) and is of note because the two genera differ vastly morphologically. Correa is unique in the Eriostemon group in possessing consistently 4-merous flowers that are mostly tubular and bird-pollinated (except for C. alba Andrews, which is nested among bird-pollinated species) (Armstrong 1979; French et al. 2016). Although bird pollination occurs in other taxa (e.g. some Leionema, Nematolepis and Philotheca), the flowers of all other genera in the group are 5-merous (with the exception of Philotheca virgata (Hook.f.) Paul G.Wilson). Correa is also distinct in possessing leaves that are oppositely arranged, although this feature also occurs in Myrtopsis. All other members of the Eriostemon group have alternate leaves. Consequently, the order of relationships among Correa, Leionema and Myrtopsis that we have recovered, (Myrtopsis (Correa, Leionema )), implies either a single transition to opposite leaves at the stem of the clade and a subsequent reversal to alternate leaves in Leionema, or separate independent transitions to opposite leaves in Myrtopsis and Correa. The placement of the New Caledonian endemic Myrtopsis firmly within the Eriostemon group is consistent with several previous studies (Bayly et al. 2013; Appelhans et al. 2021; Baker et al. 2022; Duretto et al. 2023).

Historically, the relationship of Myrtopsis with other genera of Austral-Pacific Rutaceae has been difficult to deduce from morphology. On the basis of morphological cladistic analyses, Armstrong (1991) resolved the genus as sister to the Eriostemon group (minus Correa, which was placed in the Boronia clade), although this placement was equivocal owing to limited availability of plant material. This contrasted with Hartley (1995), who considered Myrtopsis most closely related to Boronella Baill. (= Boronia Sm.), Euodia, Brombya and Medicosma, and separated the genus from the rest of tribe Boronieae on the basis of embryo shape and cotyledon width. Certain elements of the distinct morphology of Myrtopsis could be related to adaptation to New Caledonian habitats with nutrient-poor, ultramafic soils and comparatively higher temperatures and humidity than in the southern half of Australia, where the majority of species in the Eriostemon group occur.

The presence of Myrtopsis only in New Caledonia is interesting biogeographically. Evidence exists for possible land connections between Australia and New Caledonia into the Paleocene and Eocene ~60–35 million years ago (Ladiges and Cantrill 2007), meaning that a vicariance explanation cannot be ruled out should the divergence of Myrtopsis pre-date this. Time-calibrated phylogenies have tentatively estimated the divergence of Myrtopsis from the Oligocene to the mid-Miocene ~30–15 million years ago (Bayly et al. 2013; Joyce et al. 2023), implicating long-distance dispersal to New Caledonia from Australia after re-emergence of the main island during the Paleocene–Eocene (37 ± 3 million years ago; Grandcolas 2017). This is in line with the crown ages of most New Caledonian clades across the tree of life (Nattier et al. 2017). However, Bayly et al. (2013) stated that their divergence-date estimates should be treated with caution because of a limited availability of fossils within Rutaceae for calibration; they highlighted that trial analyses conducted with fewer fossil calibrations had the effect of producing much younger age estimates. Hence, the divergence of Myrtopsis in that estimation may appear younger than it should, and a vicariance explanation for the split should not yet be discounted on the basis of the current understanding of phylogenetic relationships and lack of consensus from divergence-dating analyses.