More than cowslips: Primulaceae goes tropical

Part I: Tropical primulas

Primulaceae are well known in the northern hemisphere, especially the spring flowers primrose (Primula vulgaris), cowslip (Primula veris), and oxlip (Primula elatior). These are not only charismatic wild flowers but important in horticulture, alongside several other genera including Androsace, Dodecatheon, Lysimachia, and of course Cyclamen. 

Traditionally, Primulaceae contained only these temperate herbaceous groups, and was a family of around 20 genera and 900 species. It was known to be very closely related to Myrsinaceae (Judd et al. 1994) – a group of around 40 genera and 1,700 species – but was kept separate, primarily on account of Myrsinaceae being woody and tropical. However, better understanding of angiosperm evolution through the use of DNA evidence showed that these groups were very closely related. Rather than just being a problem of rank and of where to draw the line and delimit the family, there was the additional complication that DNA evidence placed several of the traditional Primulaceae within Myrsinaceae.

Both groups have a suite of similar morphological characters and it was clear that there was little point in keeping them as separate families. The enlarged Primulaceae now comprises around 60 genera and 2,700 species in four subfamilies:

The advantages of having this larger family are that it reflects evolutionary history and it is now easier for UK and temperate-based tropical botanists to convey to non-botanists in education, teaching etc. that these groups are related to plant names and concepts embedded in the western temperate botanical lexicon. The disadvantages are that the many tropical botanists who have learnt a unique tropical plant group have now had it subsumed into a ‘European’ garden plant group and lost the immediate clarity of using a single name for this group (that is ‘Myrsinaceae’ has become Primulaceae subfamily Myrsinoideae).

Part II: Island endemism in the Indian Ocean - recent evolution and 'ecological ruin'

In 2008, I was hanging from a pulley on a zip-wire between two mountain peaks in the south-west corner of Mauritius, with a camera, a backpack full of sampling equipment, my plant press and collector book. I was determined to reach one of the only surviving trees of Badula crassa in its precarious fragment of native dry forest on the side of Le Morne mountain, and collect samples for my research.

A handful of Badula species are found in Mauritius and Reunion to the west, and a single species (Badula balfouriana) has been described from Rodrigues, to the east. Badula therefore represents one of the few endemic genera that occurs throughout the young (<15 million year old; Myr) volcanic trio of islands that comprise the Indian Ocean’s Mascarene archipelago. I had encountered another of the Mauritian Badula species (B. reticulata) in a clearing among dense stands of invasive alien shrubs, where three of the five known plants stood. Spindly stems arched towards the dark forest floor with broad dark green leaves borne in clusters towards the tips. On one tree, a long pendulous inflorescence of pink flower buds hung down resembling a string of delicate pink coral. Less than half a mile away, in a Conservation Management Area of tropical Phillipia and Phylica heath, a few scattered plants of Badula platyphylla grew on the orange laterite soil, with knobbly-elbow branches and similar, but much thicker leaves to its forest neighbour and shorter, sturdier inflorescences.

Back in the lab, I extracted DNA from the carefully collected leaf samples of all Rodriguan and Mauritian Badula species and began to build a picture of the evolutionary histories of these plants through comparison of DNA sequences. Joeri Strijk, then based in Toulouse, had sampled some of the less clearly characterised species that grew in the upland forests of Reunion. Combining our data, we searched for genetic evidence for the species described in the taxonomic literature (Coode, 1976, 1979, 1981), and geographical patterns that might explain the origin of these curious plants.

Our research recovered evidence for genetic clustering of species that are endemic to single islands, suggesting that ancestral Badula became established through 'stepping-stone' dispersal leading to single/few colonisation events on each of the Mascarene islands, followed by speciation (Bone et al., 2012; Strijk et al., 2014). Molecular clock analyses suggested that this occurred in an east to west direction (Strijk et al., 2014), from older islands (Rodrigues/ Mauritius; 10/ 8 Myr) to the youngest (Reunion; 2 Myr). An absence of clear morphological distinction among some Reunion Badula species was reflected by unclear genetic relationships, suggesting that these taxa may be hybridising, or still undergoing speciation.

While the differing geological histories of the Mascarene islands have no doubt shaped the past evolutionary trajectories of Badula (and the Mascarene flora), the varying levels of anthropogenic habitat destruction and ‘ecological ruin’ among the islands (Cheke & Hume, 2008) dramatically alter the availability of habitats that can be occupied by its descendants (Bone et al., 2012). One cannot underestimate the effect of this changed environment on the evolutionary potential of these species when Badula crassa, like many Mascarene endemic taxa, survives only on isolated peaks.