New insights into the evolution of the fig

Submitted by Tina on 8 May 2018.

The fig family (Moraceae) has fascinated botanists for generations. The family shows an extraordinary diversity in flower form and mode of pollination. We recently examined the reproductive biology of two highly unusual and poorly known relatives of the fig: Dorstenia contrajerva, native to Mexico, Central America, the Caribbean and South America, and D. lujae, native to tropical Africa. Our paper (NJB-01832) describes the floral biology of these elusive species which, along with other fig relatives, may provide novel insights into how their relative, the fig, may have evolved.

The poorly known genus Dorstenia has a remarkable diversity of floral forms, such as D. acangatara (top left) native to Brazil (photo by Anderson FP Machado), D. lujae (bottom left) native to Africa, and D. gigas, a succulent tree native to Socotra Island in Yemen (photo by Tracy Misiewicz).

The fig family contains several economically and ecologically important plants such as the fig (Ficus), mulberry (Morus) and breadfruit (Artocarpus). Figs are also among the earliest domesticated crops and have been cultivated for over 11,000 years. The biology of most relatives of the fig remains completely unexplored and could offer insight into the evolution of these economically and ecologically important plants.

Although data on obscure, tropical fig relatives is lacking, there have been extensive studies into the reproductive biology of the fig itself. Figs are intimately associated with specific species of wasp, with which they have evolved together for at least 60 million years. The familiar fig is, botanically speaking, actually a syconium – an infolded structure within which the tiny flowers are enclosed, which never see the light of day. Fig wasps (Agaonidae: Hymenoptera) crawl into the fig through a tight passageway called an ostiole, into a central cavity where the flowers are produced. They carry pollen from another fig and fertilise some of the tiny flowers, whilst laying their eggs in others. Flowers which receive an egg develop a gall which the wasps’ grubs feed on; meanwhile flowers which receive pollen develop seeds. Emergent wingless male wasps mate with winged females, which collect pollen as they crawl around the inside of the fig; the fertilised and pollen-laden female wasps then bite their way out, and fly off to find new figs to lay their eggs in.

Our hypothesis that Dorstenia may be reminiscent of an intermediate form in the evolution of this fascinating fig-fig wasp symbiosis is corroborated by a fascinating recent observation, by scientists, of flies laying eggs in the Dorstenia flowers. This exciting observation adds weight to the hypothesis that egg-laying by insects is a common trait that predated the origin of the intimate fig-wasp association that has now become a textbook example of co-evolution. 

Cross section diagrams of poorly known living fig relatives Dorstenia (left) and Antiaropsis (centre) which may be reminiscent of intermediate forms in the evolutionary pathway that led to the fig syconium (right) – a highly complex and unusual structure that is intimately associated with pollinating fig-wasps. 

Chris Thorogood (author) is Head of Science and Public Engagement for the University of Oxford Botanic Garden and Arboretum. Chris’s research interests centre on evolutionary genetics, plant taxonomy and biodiversity hotspots. Specifically he is interested in speciation and adaptive radiations in cryptic parasitic and carnivorous plant groups, as well as taxonomic diversity in biodiversity hotspots including the Mediterranean Basin region and Japan. Chris won a scholarship in 2005 to carry out his PhD research on speciation in parasitic plants at the University of Bristol for which he won the Irene Manton Prize for botany in 2009; Chris is a Fellow of the Linnean Society of London.