Friday, January 28, 2011


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Sharks are unable to distinguish colors, even though their close relatives rays and chimaeras have some color vision, according to new research by Dr. Nathan Scott Hart and colleagues from the University of Western Australia and the University of Queensland in Australia. Their study shows that although the eyes of sharks function over a wide range of light levels, they only have a single long-wavelength-sensitive cone type in the retina and therefore are potentially totally color blind.

Hart and team's findings are published online in Springer's journal Naturwissenschaften – The Science of Nature.

“This new research on how sharks see may help to prevent attacks on humans and assist in the development of fishing gear that may reduce shark bycatch in long-line fisheries. Our study shows that contrast against the background, rather than colour per se, may be more important for object detection by sharks. This may help us to design long-line fishing lures that are less attractive to sharks as well as to design swimming attire and surf craft that have a lower visual contrast to sharks and, therefore, are less ‘attractive’ to them,” said Prof. Hart.

Sharks are efficient predators and their evolutionary success is thought to be due in part to an impressive range of sensory systems, including vision. To date, it is unclear whether sharks have color vision, despite well-developed eyes and a large sensory brain area dedicated to the processing of visual information. In an attempt to demonstrate whether or not sharks have color vision, Hart and colleagues used a different technique - microspectrophotometry - to identify cone visual pigments in shark retinas and measure their spectral absorbance.

They looked at the retinas of 17 shark species caught in a variety of waters in both Queensland and Western Australia. Rod cells were the most common type of photoreceptor in all species. In ten of the 17 species, no cone cells were observed. However, cones were found in the retinae of 7 species of shark from three different families and in each case only a single type of long-wavelength-sensitive cone photoreceptor was present. Hart and team's results provide strong evidence that sharks possess only a single cone type, suggesting that sharks may be cone monochromats, and therefore potentially totally color blind.

The authors conclude: "While cone monochromacy on land is rare, it may be a common strategy in the marine environment. Many aquatic mammals − whales, dolphins and seals − also possess only a single, green-sensitive cone type. It appears that both sharks and marine mammals may have arrived at the same visual design by convergent evolution, in other words, they acquired the same biological trait in unrelated lineages."

(Photo: Springer)



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Citrus species are among the most important fruit trees in the world. Citrus has a long history of cultivation, often thought to be more than 4,000 years. Until now, however, the exact genetic origins of cultivated citrus such as sweet orange (Citrus sinensis), lemon (C. limon), and grapefruit (C. paradisi) have been a mystery.

A team of researchers from China has published a study in the Journal of the American Society of Horticultural Science that provides genetic evidence of the origins of a variety species of today's cultivated citrus.

The research team, led by Zhiqin Zhou from Southwest University, analyzed amplified fragment length polymorphism (AFLP) fingerprints—a technique that has been used successfully to assess the origin of potato cultivars—with chloroplast DNA (cpDNA) sequence analysis and nuclear internal transcribed spacer. "The combination of nuclear DNA and cpDNA data allowed us to identify the exact genetic origin of the cultivated citrus", they wrote.

The results proved that bergamot and lemon were derived from citron and sour orange, and grapefruit was a hybrid that originated from a cross between pummelo and sweet orange. The data demonstrated that sweet orange and sour orange were hybrids of mandarin and pummelo, while rough lemon was a cross between citron and mandarin. The evidence also confirmed that bergamot was a hybrid of sour orange and citron, with sour orange as the maternal parent and citron as the paternal parent.

"Our molecular evidence presented more convincing data than all other previous studies in supporting the origin of lime", noted the scientists. The data confirmed a species of Papeda to be the female parent and C. medica as the male for mexican lime.

The researchers said that a clear understanding of the citrus genetic background is necessary for better characterization and utilization of citrus germplasm, adding that this research will provide important new information for future study on the genetics and breeding of citrus.

(Photo: Xiaomeng Li)

American Society of Horticultural Science


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Planet X, an often-sought 10th planet, is so far a no-show, but Sukanya Chakrabarti has high hopes for finding what might be called Galaxy X – a dwarf galaxy that she predicts orbits our Milky Way Galaxy.

Many large galaxies, such as the Milky Way, are thought to have lots of satellite galaxies too dim to see. They are dominated by "dark matter," which astronomers say makes up 85 percent of all matter in the universe but so far remains undetected.

Chakrabarti, a post-doctoral fellow and theoretical astronomer at the University of California, Berkeley, has developed a way to find "dark" satellite galaxies by analyzing the ripples in the hydrogen gas distribution in spiral galaxies. Planet X was predicted – erroneously – more than 100 years ago based on perturbations in the orbit of Neptune.

Earlier this year, Chakrabarti used her mathematical method to predict that a dwarf galaxy sits on the opposite side of the Milky Way from Earth, and that it has been unseen to date because it is obscured by the intervening gas and dust in the galaxy's disk. One astronomer has already applied for time on the Spitzer Space Telescope to look in infrared wavelengths for this hypothetical Galaxy X.

"My hope is that this method can serve as a probe of mass distribution and of dark matter in galaxies, in the way that gravitational lensing today has become a probe for distant galaxies," Chakrabarti said.

Since her prediction for the Milky Way, Chakrabarti has gained confidence in her method after successfully testing it on two galaxies with known, faint satellites.

"This approach has broad implications for many fields of physics and astronomy – for the indirect detection of dark matter as well as dark-matter dominated dwarf galaxies, planetary dynamics, and for galaxy evolution driven by satellite impacts," she said.

Chakrabarti's colleague Leo Blitz, a UC Berkeley professor of astronomy, said that the method could also help test an alternative to dark matter theory, which proposes a modification to the law of gravity to explain the missing mass in galaxies.

"The matter density in the outer reaches of spiral galaxies is hard to explain in the context of modified gravity, so if this tidal analysis continues to work, and we can find other dark galaxies in distant halos, it may allow us to rule out modified gravity," he said.

The Milky Way is surrounded by some 80 known or suspected dwarf galaxies that are called satellite galaxies, even though some of them may just be passing through, not captured into orbits around the galaxy. The Large and Small Magellanic Clouds are two such satellites, both of them irregular dwarf galaxies.

Theoretical models of rotating spiral galaxies, however, predict that there should be many more satellite galaxies, perhaps thousands, with small ones even more prevalent than large ones. Dwarf galaxies, however, are faint, and some of the galaxies may be primarily invisible dark matter.

Chakrabarti and Blitz realized that dwarf galaxies would create disturbances in the distribution of cold atomic hydrogen gas (H I) within the disk of a galaxy, and that these perturbations could reveal not only the mass, but the distance and location of the satellite. The cold hydrogen gas in spiral galaxies is gravitationally confined to the plane of the galactic disk and extends much farther out than the visible stars – sometimes up to five times the diameter of the visible spiral. The cold gas can be mapped by radio telescopes.

"The method is like inferring the size and speed of a ship by looking at its wake," said Blitz. "You see the waves from a lot of boats, but you have to be able to separate out the wake of a medium or small ship from that of an ocean liner."

The technique Chakrabarti developed involves a Fourier analysis of the gas distribution determined by high-resolution radio observations. Her initial predication of Galaxy X around the Milky Way was made possible by a wealth of data already available on the atomic hydrogen in our galaxy. To test her theory on other galaxies, she and her collaborators used recent data from a radio survey called The HI Nearby Galaxy Survey (THINGS), conducted by the Very Large Array, as well as its extension to the Southern Hemisphere, THINGS-SOUTH, a survey carried out by the Australia Telescope Compact Array.

''These new high-resolution radio data open up a wealth of opportunities to explore the gas distributions in the outskirts of galaxies'', said co-author Frank Bigiel, a UC Berkeley post-doctoral fellow who is also co-investigator of the THINGS and THINGS-SOUTH projects.

Collaborating with Bigiel and Phil Chang of the Canadian Institute of Theoretical Astrophysics, Chakrabarti looked at data for the Whirlpool Galaxy (M51), which has a companion galaxy one-third the size of M51, and NGC 1512, with a satellite one-hundredth the size of the galaxy. Her mathematical analysis correctly predicted the mass and location of these satellites.

She said her technique should work for satellite galaxies as small as one-thousandth the mass of the primary galaxy.

Chakrabarti predicted the mass of Galaxy X, for example, to be one-hundredth that of the Milky Way itself. Based on her calculations with Blitz, the galaxy currently sits across the Milky Way somewhere in the constellations of Norma or Circinus, just west of the galactic center in Sagittarius when viewed from Earth.

She contrasts her prediction of Galaxy X with previous arguments for a Planet X beyond the orbit of Neptune. In the 19th century, what would have been at the time a ninth planet was proposed by famed astronomer Percival Lowell, but his prediction turned out to be based on incorrect measurements of Neptune's orbit. In fact, Pluto and other objects in the Kuiper Belt, where the planet was predicted to reside, have masses far too low to exert a measurable gravitational effect on Neptune or Uranus, Chakrabarti said. Since then, perturbations in the orbits of other bodies in the solar system have set off periodic searches for a 10th planet beyond the now "dwarf" planet Pluto.

On the other hand, Galaxy X – or a satellite galaxy one-thousandth the mass of the Milky Way – would still exert a large enough gravitational effect to cause ripples in the disk of our galaxy.

Barbara Whitney, a Wisconsin-based astronomer affiliated with the Space Sciences Institute in Boulder, Colo., hopes to target Galaxy X as part of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) conducted with the Spitzer Space Telescope.

Chakrabarti and Blitz also calculated that the predicted galaxy is in a parabolic orbit around the Milky Way, now at a distance of about 300,000 light years from the galactic center. The galactic radius is about 50,000 light years.

"Our paper is a proof of principle, but we need to look at a much larger sample of spiral galaxies with optically visible galactic companions to determine the incidence of false positives," and thus the method's reliability, Chakrabarti said.

(Photo: Sukanya Chakrabarti/UC Berkeley)

University of California, Berkeley




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