The humongous fruit, the tiny pollinator and the duplicitous fungus

By Athayde Tonhasca

Among the range of exotic fruits available to us on grocery stalls, we are not likely to find jackfruit (Artocarpus heterophyllus). But that could change, as the worldwide cultivation and consumption of the fruit have been increasing steadily, fuelled in part by its use as a meat and starch substitute. Jackfruit is a source of dietary fibre and it’s high in potassium and vitamin B. The pulp can be eaten fresh, dried or roasted; seeds can be boiled, roasted or ground into flour. Pulp and seeds can be turned into soups, chips, jams, juices, and ice cream. The jack tree requires little care once it’s been established, and produces high-quality, rot-resistant timber used for furniture and musical instruments. The species is cauliflorous (flowers and fruits grow from trunks and large branches), yielding 150-200 fruits per year. The jackfruit is the largest of all tree-borne fruits, reaching up to 60 cm in length and weighing up to 50 kg. You don’t want to take a nap under a jack tree.

A jack tree loaded with fruit © Crops for the Future, an opened fruit © Kinglaw, and jackfruit seeds and flesh © Aznaturalist. Wikimedia Commons.

Because of all these good points, the jackfruit is frequently cited as a potential contributor to food security throughout the tropics, especially considering the tree’s capacity to withstand high temperatures and droughts. But despite the growing interest in jackfruit, we know little about its reproduction. 

The jack tree is monoecious, that is, it has male and female flowers on separate inflorescences. We are not sure how pollen is transferred between flowers: wind, some beetles and flies have been proposed as possible pollinating agents. Researchers in Florida, USA, found an unexpected candidate: a hitherto unknown species of gall midge (family Cecidomyiidae), Clinodiplosis ultracrepidata (Gardner et al., 2018. International Journal of Plant Sciences 179: 350-367). These midges are attracted by the sweet scent released by the flowers. A midge may get some pollen attached to it when visiting a male flower, and a subsequent visit to a female flower could result in pollination. 

A female C. ultracrepidata and a grain of pollen adhering to the abdomen of a midge © Gardner et al., 2018. International Journal of Plant Sciences 179: 350-367.

But there’s a twist in this relationship: pollination by C. ultracrepidata may depend on the jack tree being sick.    

The fungus Rhizopus artocarpi is a common fungal disease of jackfruit flowers and fruit. It initially infects male flowers, later spreading as a greyish growth of mycelia. The fungus advances slowly until the whole inflorescence or young fruit rot and fall off. Fruit rot, as the disease is known, can cause total loss of fruit if conditions are right for fungus development.

A fruit (A) and a male inflorescence (B) infected with Rhizopus sp. © Ghosh et al., 2015. Biological Control 83: 29-36.

Fruit rot could be bad news if unchecked, but some fungus-covered inflorescences are needed by female midges to lay their eggs and by their larvae to feed and develop. Absence of fungal infestation may break the reproductive cycle of C. ultracrepidata, with unknown consequences for jack tree pollination.

This tale illustrates important facts about pollination. First, we have only the vaguest idea of the players involved; C. ultracrepidata likely originated from Asia, sneaked into America by accident, and remained unknown to science until someone bumped into it. There must be countless other anonymous species quietly pollinating crops and wild plants. Second, we have a partial, often highly speculative understanding of processes. For example, we don’t know how relevant C. ultracrepidata is for the pollination of jack tree flowers in Florida or elsewhere: the study did not rule out other insects or wind as contributors. We also don’t know the equilibrium point in this tripartite relationship, i.e., what’s the level of fungal infestation that’s tolerable for a jack tree and sufficient for midge reproduction.  

There is much to be learned in the field of pollination ecology. Considering its relevance to food security, the economy, and to our health, questions about pollination mechanisms and pollinating agents are worth pursuing.  

Jackfruit for sale in New York City. You may be needing a bigger bag © EdwinAmi, Wikimedia Commons.