A puzzle with uniform flowers

Normally you would expect organs to change across evolutionary time. However, this is not always the case, and some clades can be species-rich, but still have very similar flowers.

Flower morphology is often thought to be connected with speciation. If the flower changes shape, then different pollinators have access and so genes mix less between differing members of the same species until they become two different species. Annals of Botany had an article looking at this. It makes sense as an explanation. Plants with the same flowers mate with each other. So while it’s not a surprise to go for walk and see lots of flowers that look the same, it is a surprise when you find they’re all different species. In a study in New Phytologist Thais Vasconcelos and colleagues describe the flowers of Myrcia (Myrtaceae), a genus with over 700 species with remarkably similar flowers.

Floral similarity across the Myrcia phylogeny.
Floral similarity across the Myrcia phylogeny. Image from Vasconcelos et al. 2018

Myrcia are found in the Neotropics, between Mexico and South America. They’re difficult to describe, as while the flowers might be similar, the plants are extremely diverse. The authors say: “Growth habit varies from small subshrubs of c. 10 cm to trees of 40 m, sometimes even in closely related species…”

The flowers are geared to generalist pollinators, so there’s always hope for a suitable pollinator to be around, which makes speciation a puzzle. Vasconcelos and colleagues propose allopatry as a model for speciation. Seeds are carried long distances and isolated from parent populations. Geographically isolated, populations can speciate and then other factors can cause more differences. say: “[T]here is evidence for high levels of diversity of chemical compounds in Myrcia leaves…, reflecting selective pressure from herbivores and natural enemies that is very strong in tropical areas.” The authors compare the ability of Myrcia to adapt traits to differing conditions to Croton (Euphorbiaceae).

The authors argue that Myrcia developed an optimal pollination system early on in their development, so there is not the same opportunity for pollinator-led speciation that other genera have. In this situation, the evidence of Myrcia shows that other speciation processes are in action for some plants.

Reference List

Van der Niet, T., Peakall, R., & Johnson, S. D. (2014). Pollinator-driven ecological speciation in plants: new evidence and future perspectives. Annals of Botany, 113(2), 199–212. https://doi.org/10.1093/aob/mct290

Vasconcelos, T. N. C., Chartier, M., Prenner, G., Martins, A. C., Schönenberger, J., Wingler, A., & Lucas, E. (2018). Floral uniformity through evolutionary time in a species-rich tree lineage. New Phytologist. https://doi.org/10.1111/nph.15453

Vasconcelos, T. N. C., Lucas, E. J., Faria, J. E. Q., & Prenner, G. (2017). Floral heterochrony promotes flexibility of reproductive strategies in the morphologically homogeneous genus Eugenia (Myrtaceae). Annals of Botany, 121(1), 161–174. https://doi.org/10.1093/aob/mcx142

Staggemeier, V. G., Diniz-Filho, J. A. F., Forest, F., & Lucas, E. (2015). Phylogenetic analysis in Myrcia section Aulomyrcia and inferences on plant diversity in the Atlantic rainforest. Annals of Botany, 115(5), 747–761. https://doi.org/10.1093/aob/mcv005

Arévalo, R., van Ee, B. W., Riina, R., Berry, P. E., & Wiedenhoeft, A. C. (2017). Force of habit: shrubs, trees and contingent evolution of wood anatomical diversity usingCroton(Euphorbiaceae) as a model system. Annals of Botany, mcw243. https://doi.org/10.1093/aob/mcw243