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Did you hear? The genome of the banana has been sequenced, an important development in scientist's efforts to produce better bananas. 

A look at that genome has revealed curious things, said Pat Heslop-Harrison, a plant geneticist at the University of Leicester in England who was a coauthor of the report published this week in the journal Nature. 

For example, there are regions of the banana genome that don't seem to be involved in making proteins but are shared by many different species of plants, far beyond bananas. What, he wonders, are they doing? 

There are remnants of bits of banana streak virus spliced into the banana genome (too broken-up to cause disease, however). 

There are whole sets of DNA repeats that plants normally have but bananas do not. And, intriguingly, three times since this genus of giant herbs took an evolutionary turn away from its relatives -- the grasses -- it has duplicated its entire set of chromosomes. 

Two of the doublings took place at the Cretaceous-Tertiary boundary 65 million years ago, back when the dinosaurs and lots of other species went extinct, Heslop-Harrison noted. 

Duplications like this are known to have happened in other plant groups at this same time but haven't occurred since, Heslop-Harrison said. Scientists don't know why, but they believe having extra copies of genes may have imparted some stability to plants during a time of rapid climate change after an asteroid hit Earth.

Having more than one gene of each type means that if one gene of a set loses function, the plant still has another one that works. And there's more room for adaptability to new circumstances, because one gene could be altered and co-opted for new purposes and there would still be the other one left to perform the original job. 

"Perhaps it's the reason [bananas have] done so well in the subsequent millions of years," Heslop-Harrison said. "One can ask, will changes occurring in the world's climate now mean there's going to be a whole set of new genome duplications that will enable plants to survive? We don't know that, but it's interesting to consider." 

The banana genome sequenced by the French scientists was from the Pahang, a wild Malaysian banana of the species Musa acuminata. It's a key species in the complicated evolution of the bananas and plantains people eat around the world, including the Cavendish banana that we buy at the supermarket. 

The sterile Cavendish is a so-called triploid: It has three sets of chromosomes instead of the normal two. One of those genomes came from Pahang. The others came from other subspecies of Musa acuminata. 

The changes occurred stepwise, and went something like this:
  • Thousands of years ago, two wild banana species from different parts of the islands of Southeast Asia were brought into the same range by people. They formed hybrids. A bit like mules, the hybrids were vigorous but fairly sterile.
  • The hybrids were kept going without through propagation of their shoots.
  • At some point, the hybrids developed the ability to set fruit without being fertilized.
  • Then (for most bananas, including the Cavendish) came another chance event that caused the hybrids to end up with three sets of chromosomes. Every now and again, the few viable eggs and pollen that they made would mistakenly contain two sets of chromosomes instead of just one.



When a double-chromosome pollen combined with a single-chromosome egg (or vice versa), the result was a hopelessly sterile plant with even more vigorous fruit.

Events like this happened more than once and sometimes included other types of ancestral banana species. 

Some scientists, in fact, have made a whole study of banana domestication and movement around the world. They've pieced the story together using quite different strands of information, including the genomes of wild and cultivated bananas, the microscopic relics of banana leaf material found at archaeological sites, and even the word for "banana" in different languages. 
ادامه مطلب


Map of Eurasia

Pastoralism is a lifestyle in which economic activity is based primarily on livestock. Archaeological evidence suggests that by 3,000 B.C., and perhaps even earlier, there had emerged on the steppes of Inner Eurasia the distinctive types of pastoralism that were to dominate the region’s history for several millennia. Here, the horse was already becoming the animal of prestige in many regions, though sheep, goats, and cattle could also play a vital role. It is the use of horses for transportation and warfare that explains why Inner Eurasian pastoral-ism proved the most mobile and the most militaristic of all major forms of pastoral-ism. The emergence and spread of pastoral-ism had a profound impact on the history of Inner Eurasia, and also, indirectly, on the parts of Asia and Europe just outside this area. In particular, pastoral-ism favors a mobile lifestyle, and this mobility helps to explain the impact of pastoralist societies on this part of the world.

 

The mobility of pastoralist societies reflects their dependence on animal-based foods. While agriculturalists rely on domesticated plants, pastoralists rely on domesticated animals. As a result, pastoralists, like carnivores in general, occupy a higher position on the food chain. All else being equal, this means they must exploit larger areas of land than do agriculturalists to secure the same amount of food, clothing, and other necessities. So pastoralism is a more extensive lifeway than farming is. However, the larger the terrain used to support a group, the harder it is to exploit that terrain while remaining in one place. So, basic ecological principles imply a strong tendency within pastoralist lifeways toward nomadism (a mobile lifestyle). As the archaeologist Roger Cribb puts it, The greater the degree of pastoralism, the stronger the tendency toward nomadism.” A modern Turkic nomad interviewed by Cribb commented: The more animals you have, the farther you have to move.”

 

Nomadism has further consequences. It means that pastoralist societies occupy and can influence very large territories. This is particularly true of the horse pastoralism that emerged in the Inner Eurasian steppes, for this was the most mobile of all major forms of pastoralism. So, it is no accident that with the appearance of pastoralist societies there appear large areas that share similar cultural, ecological, and even linguistic features. By the late fourth millennium B.C., there is already evidence of large culture zones reaching from Eastern Europe to the western borders of Mongolia. Perhaps the most striking sign of mobility is the fact that by the third millennium B.C., most pastoralists in this huge region spoke related languages ancestral to the modern Indo-European languages. The remarkable mobility and range of pastoral societies explain, in part, why so many linguists have argued that the Indo-European languages began their astonishing expansionist career not among farmers in Anatolia (present-day Turkey), but among early pastoralists from Inner Eurasia. Such theories imply that the Indo-European languages evolved not in Neolithic (10,000 to 3,000 B.C.Anatolia, but among the foraging communities of the cultures in the region of the Don and Dnieper rivers, which took up stock breeding and began to exploit the neighboring steppes. 

Nomadism also subjects pastoralist communities to strict rules of portability. If you are constantly on the move, you cannot afford to accumulate large material surpluses. Such rules limit variations in accumulated material goods between pastoralist households (though they may also encourage a taste for portable goods of high value such as silks or jewelry). So, by and large, nomadism implies a high degree of self-sufficiency and inhibits the appearance of an extensive division of labor. Inequalities of wealth and rank certainly exist, and have probably existed in most pastoralist societies, but except in periods of military conquest, they are normally too slight to generate the stable, hereditary hierarchies that are usually implied by the use of the term class. Inequalities of gender have also existed in pastoralist societies, but they seem to have been softened by the absence of steep hierarchies of wealth in most communities, and also by the requirement that women acquire most of the skills of men, including, often, their military skills.


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