Baobab seeds and plants floated from Madagascar to mainland Africa and as far as Australia

Credit: CC0 Public domain

There are eight species of baobab in the world and their distribution is surprising. Six are on Madagascar, an island in the Indian Ocean; we find one on the African continent; and the last one is far away in Australia.

The origin of this group of plants has long fascinated people. I joined a global team of plant genetics and genomics researchers led by Tao Wan and Qing Feng Wang from Wuhan Botanic Gardens in China and Ilia Leitch from the Royal Botanic Gardens, Kew in London to sequence, assemble and study the genomes of eight baobabs. species.

Baobabs are remarkable keystone species, capable of supporting a wide diversity of animals for food and shelter. They can reach enormous dimensions (depending on the species) both in height and in diameter. Larger species can dominate understory vegetation. When you encounter a large African baobab tree alone, it truly resembles the “house tree” in the movie Avatar, not only because of its size, but also because it is home to a wide variety of life.

Through in-depth analysis of genomic data, we were able to explore the genetic diversity of each species. This is important because it may indicate the resilience of each species to some of the major environmental challenges that baobabs face, such as land use issues and climate change, and therefore their risk of extinction.

Our research revealed that the baobab trees we see today are all native to Madagascar. They began to evolve into distinct species around 21 million years ago. Later, two of these species traveled to Africa and Australia before the lineages became extinct in Madagascar.

The evolutionary history of baobabs

In the genomes of baobabs, we found evidence of ancient hybridizations between species. This means they must all have lived together in Madagascar at some point and crossed paths. By adding geological and paleoclimatic data, we were able to calculate that in Madagascar, different species of baobab evolved, influenced by ice ages and sea levels that rose and fell over millions of years.

The African species (Adansonia digitalate), like the Australian species (Adansonia gregorius and also known as “boab”), almost certainly left Madagascar as seeds or seedlings, floating on piles of debris clumped together like rafts, which were carried out to sea by rivers swollen by lightning storms . The first African pioneer of the baobab probably arrived within the last 12 million years.

From there, its numbers grew, often with the help of elephants eating its seeds. Many baobab seeds pass through elephants undamaged and are deposited in dung piles, up to 65 km from where the elephant ate the baobab fruit.

We believe that by the time baobabs spread across the continent to West Africa, a cell division error had occurred in the generation of pollen or eggs, leading to an increase in number of chromosomes in their cells (from 88 to 168). . Such an increase in chromosome number is common in plants and is known as polyploidy. Polyploidy is a process that genetically isolates the plant from its parents. Unlike most animals, plants can self-pollinate to increase their numbers. Thus, a genetic error can, in rare circumstances, generate a new species.

The new polyploid baobab would then likely have become the dominant species in Africa, replacing its parental lineages across the continent. This could have happened over about 2,500 years, based on calculations of how fast elephants can move baobab seeds.

I say that the polyploid baobab has replaced its parents. But did it happen? We are currently studying whether the ancient form of the baobab (perhaps a form named Adansonia Kilima) still exists in Zimbabwe and on the slopes of Mount Kilimanjaro and beyond. Scientists assumed this after an analysis of baobab leaves and the number of chromosomes in those areas suggested they came from two different species of baobab. But this information remains to be verified. Further detection work is clearly needed to determine whether Adansonia Kilimathe baobab with the initial chromosome number of 88 still exists.

Their astonishing growth form

Baobabs are nicknamed “the tree of life” because they support the surrounding ecosystem and the many life forms that live in their hollow trunks, nest on and in their branches, and eat their nectar and fruit. The root system is massive and helps slow soil erosion and recycle nutrients.

Massive trunks are hollow cylinders of poor quality wood containing many living cells filled with water. Some of Australia’s largest and oldest baobab trees are estimated to hold more than 100,000 liters of water. The water-filled trunk cells generate hydrostatic pressure that gives strength to the tree (the water acting much like air in a bouncy castle).

Unfortunately for the tree, the trunk can provide water for elephants. During particularly dry seasons, trees are damaged or destroyed when elephants remove the bark to draw water from the wood. The bark is also partially resistant to fire damage, which is essential for a plant living in the savannah.

Baobab flowers are large and evolved alongside large, nocturnal, sugar-eating animals like hawkmoths, fruit bats, and Madagascar lemurs.

Why is this important

Two of Madagascar’s species are already listed as endangered, based on the International Union for Conservation of Nature’s Red List criteria. They exist in small numbers and can only survive in certain habitats, which are diminishing. They are not genetically diverse. We recommend that their threat of extinction be raised to the next level, “critically endangered”, so that they can be protected.

Our research shows that a third, even rarer species (already assessed as critically endangered) is vulnerable to hybridization, i.e. crossing with a more common species. This means that rare species could be genetically overwhelmed and lose their distinctiveness.

In practice, this may involve difficult conservation decisions, such as removing the most common baobab species where they occur together, replanting the rarest baobabs in suitable areas lacking the more common species, or simply accepting that the both species are slowly losing their distinctiveness.

Provided by The Conversation

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