Tiny plants make a huge impact

Kew researchers are collaborating with scientists worldwide in a detailed study of Hydatellaceae, a family of tiny aquatic plants that occur in Australia, New Zealand and India.

Although they are difficult to locate and tricky to examine closely, several factors make this family useful for evolutionary studies. Perhaps most importantly, they represent an early lineage of flowering plants and can therefore provide a window into their evolutionary past.

A close relationship between Hydatellaceae and water-lilies (including Nymphaea and Victoria) was discovered relatively recently, following DNA-based studies [1]. Initially, because Hydatellaceae have linear leaves and look superficially grass-like, scientists thought they were closely related to grasses, and wrongly classified them as monocotyledons. Subsequent studies, many of them coordinated at Kew, have confirmed this dramatic reclassification.

From other analyses of both DNA and fossils, we know that the water-lily lineage is among the oldest living groups of flowering plants. Therefore, the inclusion of Hydatellaceae with the water-lilies places them close to the “root” of flowering plants and potentially gives them greater significance in understanding angiosperm evolution.

One caveat is that this relatively ancient family could have developed many unique features during its long evolutionary history, and may not even remotely resemble ancestral angiosperms. However, science depends on assessment of evidence from a wide range of sources. Their placement near the base of the angiosperm family tree means that Hydatellaceae could provide a much-needed yardstick for comparison with other flowering plants. 

A number of Hydatellaceae species are grown at Kew, and we use various different types of microscope to study many aspects of their structure. One strange feature soon caught our attention in Trithuria submersa and other species with bisexual flowers [5]. Their flowers are “inside-out”, because the carpels (female organs) surround the stamens (male organs). In contrast, one of the primary characteristics of almost all other flowers is that the carpel zone is located in the centre, surrounded by the stamens. The stamens produce the pollen and the carpels bear the ovules. The genetic basis for the highly conserved pattern found in most flowers is increasingly well understood.

So, Trithuria submersa appears to have extremely unusual flowers. Could this mean that its reproductive units are not actually flowers? In all other respects they look like flowers: they have a radially whorled structure with a ring of sterile petal-like organs surrounding the stamens and carpels. In fact, current evidence remains ambivalent as to whether these “non-flowers” of Hydatellaceae should correctly be interpreted as flowers or inflorescences [6,7].

They could represent a structure in which typical floral patterning has been ‘lost’ during the course of evolution. Alternatively, given the phylogenetic placement of Hydatellaceae close to the base of the angiosperms, they could represent the sole survivor of an early experiment with floral patterning that differs from the pattern that rapidly became firmly established in all other flowering plants.

Following the discovery that Hydatellaceae belongs to a basal angiosperm lineage, our knowledge of many aspects of the taxonomy, comparative and evolutionary morphology, and ecology of the family has increased exponentially. Nevertheless, several important issues remain to be studied. We are investigating other aspects of morphological evolution in this interesting family, such as the structure of the pollen and stigma [8,9], ovules [10], fruits and seeds [11,12,13] and seedlings [14,15,] including the remarkable range of cotyledon diversity. The potential impact of these studies is to provide us with a broader understanding of what the ancestral angiosperms could have looked like. Ultimately, this knowledge is crucially important in understanding how flowering plants evolved.