Discovery: Gene trigger for asexual plant reproduction

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March 20, 2016 05:07

“Our results can explain at the molecular level how asexual reproduction, known as parthenogenesis or apomixes in plants, has evolved. In these processes, genetically identical plants are formed.”




Plant

Plant. (photo credit: INGIMAGE / ASAP)

Boy meets girl, sperm merges with egg cell – or so the story goes. The process of reproduction – of creating new life on the planet – is essentially the same for humans, animals and most plants. Both female and male organisms are required to contribute.

A new joint study published in Nature Plants by researchers at Tel Aviv University and Freiburg University offers a rebuttal – the discovery of the genetic trigger for the development of offspring, without cross-fertilization, in moss. The research, led jointly by TAU Life Sciences Faculty Prof. Nir Ohad, director of the Manna Center Program for Food Safety and Security and Prof. Ralf Reski of the German university, identifies and explores the master genetic switch for self-reproduction in the moss Physcomitrella patens. They found that the BELL1 gene triggers a pathway of genes, which – without fertilization – facilitate embryo development to form fully functional adult moss plants.

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“Moss, possessing egg cells and motile sperm, serves as a simple model plant to understand self-fertilization processes,” said Ohad.

“Our results can explain at the molecular level how asexual reproduction, known as parthenogenesis or apomixes in plants, has evolved. In these processes, genetically identical plants are formed.”

In reproduction, after the fusion of sperm and egg cell, a network of genes is activated, leading to the development of an embryo, which grows into a new living being.

Until now, it was unclear whether a central genetic switch for this process existed. In their latest study, the team pinpoints the gene BELL1 as the master regulator for the formation of embryos and their development in moss.

“This gene was conserved in evolution, and our findings may have implications for agriculture by generating genetically identical offspring from high-yielding crop plants,” said Ohad, a specialist in the epigenetic regulation of reproductive development who helped identify the first BELL genes in seed plants 20 years ago as member of a team led by Prof. Robert Fischer of UC Berkeley.

The scientists harnessed genetic engineering to activate the BELL1 gene in moss plants and observed embryos developed spontaneously on a specific cell type. To their surprise, these embryos grew to fully functional moss spore capsules (sporophytes), which later formed spores and grew into new adult moss plants.

The scientists are carrying forward their research to identify the exact genes triggered by BELL1 to facilitate the formation of embryos without fertilization.

“We identified BELL1 as a master regulator for embryo development in mosses,” said noted Ohad.

“The knowledge gained from such research may help to modernize agriculture, allowing for the cloning of certain important plants and the distribution of their seeds to farmers.”

“Our results are important beyond mosses,” said Reski.

“First, they can explain how algae developed into land plants and thus shaped our current ecosystems. Second, they may help to revive the concept of genetic master regulators in the development of plants, animals and humans.”

WHY SPIDERMAN CAN’T EXIST Latest research reveals why geckos are the largest animals able to scale smooth vertical walls: larger climbers would require unmanageably large sticky footpads.

Scientists estimate that a human would need adhesive pads covering 40 percent of his or her body surface to walk up a wall like Spiderman.

They believe their insights have implications for the feasibility of large-scale, gecko-like adhesives.

A new study, published today in the journal PNAS, shows that in climbing animals, from mites and spiders to tree frogs and geckos, the percentage of body surface covered by adhesive footpads grows as body size increases.

This sets a limit to the size of animal that can use this strategy because larger animals would require impossibly big feet.

Dr. David Labonte and his zoology team at the University of Cambridge found that tiny mites use approximately 200 times less of their total body area for adhesive pads than geckos, nature’s largest adhesion-based climbers.

Humans would need about 40% of our total body surface or 80% of our front to be covered in sticky footpads if we wanted to do a convincing Spiderman impression.

Once an animal is big enough to need a substantial fraction of its body surface to be covered in sticky footpads, the necessary morphological changes would make the evolution of this trait impractical, suggested Labonte.

“If a human, for example, wanted to walk up a wall the way a gecko does, we’d need impractically large sticky feet; our shoes would need to be a European size 145 or a US size 114,” says Walter Federle, another senior zoologist.

The researchers say that these insights into the size limits of sticky footpads could have profound implications for developing large-scale bio-inspired adhesives, which are currently only effective on very small areas.

“As animals increase in size, the amount of body surface area per volume decreases. An ant has a lot of surface area and very little volume, and a blue whale is mostly volume with not much surface area,” explained Labonte.

“This poses a problem for larger climbing species because, when they are bigger and heavier, they need more sticking power to be able to adhere to vertical or inverted surfaces, but they have comparatively less body surface available to cover with sticky footpads. This implies that there is a size limit to sticky footpads as an evolutionary solution to climbing – and that turns out to be about the size of a gecko.”

Larger animals have evolved alternative strategies to help them climb, such as claws and toes to grip with. The researchers compared the weight and footpad size of 225 climbing animal species including insects, frogs, spiders, lizards and even a mammal.

“We compared animals covering more than seven orders of magnitude in weight, which is roughly the same as comparing a cockroach to the weight of Big Ben,” said Labonte.

“We were looking at vastly different animals – a spider and a gecko are about as different as a human is to an ant, but if you look at their feet, they have remarkably similar footpads,” he said.

“Adhesive pads of climbing animals are a prime example of convergent evolution, where multiple species have independently, through very different evolutionary histories, arrived at the same solution to a problem. When this happens, it’s a clear sign that it must be a very good solution.

“Our study emphasizes the importance of scaling for animal adhesion, and scaling is also essential for improving the performance of adhesives over much larger areas.

There is a lot of interesting work still to do looking into the strategies that animals have developed in order to maintain the ability to scale smooth walls, which would likely also have very useful applications in the development of large-scale, powerful yet controllable adhesives,” concluded Labonte.


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