Thinking Point
Dr Priya Davidar
Professor (Retired), Pondicherry University
Globally our crop plants have been domesticated from wild species by humans since a long time. For instance, rice and wheat were domesticated thousands of years ago. These wild relatives of crop plants still exist and are an important source of genes for crop improvement as cultivated crops have reduced genetic variation due to selection and plant breeding. These wild species provide genes that increase yield, add nutrient value and have greater resistance to pests. There are over 1000 plant species that are closely related to world’s most important food crops, and 373 species of crop wild relatives have been documented in India. Conserving wild relative of crop plants is vitally important for food security as these species are under severe threat from habitat loss, human impacts, climate change and invasive species. An added threat is the swamping of the genetic material of wild relatives by hybrid and transgenic crops.
The brinjal (Solanum melongena), also called egg plant and aubergine, is widely cultivated throughout the world. The eggplant is a native of Asia and was domesticated since a thousand years in the Indian subcontinent, China and South East Asia. There are dozens of varieties of brinjal, including local cultivars and hybrids. There are many land races in India, probably a result of local selection by farmers, which are in danger of being lost with the cultivation of hybrids.
The closest ancestor of the cultivated brinjal is the wild brinjal, a spiny perennial shrub (Solanum insanum L. or S. melongena var. insanum), that grows commonly around villages and in wastelands in drier areas of India. The wild brinjal occurs across tropical Asia and Madagascar. The flowers are violet or white in colour and smaller than that of the crop plant. Like the cultivated plant, the wild relative has a sexual system termed andro-monoecious: some flowers are purely male (with only stamens, the male part) and others are bisexual (stamens and pistil). The fruits are small, round or ovoid in shape and green and reticulated, sometimes white. The ripe fruit is yellow and seeds are dispersed by wild and domestic ungulates. The crop brinjal can set seed with its own pollen whereas the wild brinjal is mostly outcrossing.
The wild brinjal is common in the lower-elevation dry forests of the Western Ghats. In the drier areas of the Nilgiri Biosphere Reserve of the Western Ghats, where I grew up, the unripe fruits which are bitter were widely used as a vegetable by tribal communities. This is no longer the case as communities appear to favour the cultivated brinjal which is easier to cook. Wild brinjal populations which were very common even 10-years ago, are becoming rarer.
Transgenic brinjal, also called Bt brinjal, was developed in India to reduce damage by lepidopteran pests. Genes from Bacillus thuriginensis, a soil bacteria commonly used as a biological pesticide, were introduced into the genetic material of the crop plant to make it more resistant to pest attack. This can have drastic consequences if the transgene is transferred to the wild relatives through pollination, which might eventually lead to its extinction. Therefore, before transgenic crops are introduced, it’s important to check whether genes can be transferred from crop to wild relatives, through pollination.
A team led by Dr Allison Snow of the Ohio State University, an internationally acknowledged expert on risk assessment for GM crops, Dr Marie-Christine Daunay of INRA, France an expert on crop brinjal, Dr Remy Pasquet and myself together with postdoctoral researcher Dr Evans Mutegi of Ohio State University, and students from Pondicherry University undertook a study on the potential for pollen-mediated, crop-to-wild gene flow. Permits were obtained from the National Biodiversity Authority.
Our study focused on two aspects: the first was a survey of populations of wild brinjal (Solanum insanum) and whether they co-occurred with crop plants, observations of pollinator visitation patterns to wild and cultivated brinjal, to see whether there was potential for hybridization between crop and wild brinjal, through pollen transfer by shared pollinators. The second aspect was to examine genetic diversity, population structure, and outcrossing in 10 natural populations of wild brinjals, and three cultivated crop brinjals using genetic techniques.
Our survey showed that wild brinjal is a widespread perennial in southern India, found in disturbed sites such as roadsides, wastelands, and sparsely vegetated areas near villages and agricultural fields and it often co-occurred with cultivated brinjal with which it shared pollinators such as a diversity of solitary and social bees (Amegilla, Xylocopa, Nomia, and Tetragonula). Hand-crosses between wild and cultivated brinjal resulted in seed set and viable offspring. The close genetic affinity between cultivars and nearby wild/weedy brinjal at some locations indicates that natural gene flow has occurred between them via pollination, and may explain the wide range of morphological diversity observed in cultivated brinjal, and the frequent confusion between different forms. Therefore, there is a strong possibility that introduction of transgenic brinjal and the transfer of the transgene to wild brinjal could lead to unintended consequences and could result in the loss of wild germplasm.
A doctoral student of Pondicherry University, R Lekshmi examined pest attack on wild and cultivated brinjal. She showed that crop brinjal suffered 54% loss of fruits through attack by moth larvae, whereas the wild brinjal fruits were not fed upon by these larvae even in enclosed condition. This shows the importance of wild germplasm for crop improvement.
The proposal to introduce Bt brinjal in India was being advocated by scientists as a solution to counter pest attack and our study has shown that there is possibility of the transgene escaping into the wild populations. If transgenic Bt brinjal were to be introduced, it could compromise efforts to maintain wild germplasm that is “GM-free.” India is one of the countries cited as a centre of origin and domestication of the brinjal and therefore in situ conservation strategies for wild germplasm is required to protect our natural heritage. Transgene escape into wild brinjal populations could also lead to weedy infestation among cultivated brinjal, which has happened with transgenic crops such as rice, which reduces crop production and quality. Therefore, firstly identifying and protecting wild relatives of cultivated crops is among the steps taken by many countries to preserve their germplasm resources. Secondly the introduction of transgenic crops in geographical regions where their wild relatives are present is usually avoided. In situ and ex situ conservation of crop wild relatives are vital for the conservation of biodiversity and for safeguarding our food security.
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