Daily Archives: August 2, 2017

August 1, 2017 by Eric Hamilton Credit: CC0 Public Domain

After Dan Levitis and his wife lost two pregnancies, before having their three children, he was drawn to investigate why pregnancy loss is so common, and whether other living beings face the same struggle his family did.

Levitis, a scientist in the University of Wisconsin-Madison Department of Botany, had one main suspect in mind: meiosis, which organisms use to produce sperm and eggs for sexual reproduction. He describes meiosis as an intricate cellular line dance, one that mixes up chromosomes to reshuffle genes. This rearrangement helps create offspring that are different from their parents, offspring that might be better equipped to survive in a changing world.

But meiosis is also one of the most complex processes that cells undergo, and a lot can go wrong as chromosomes tangle and untangle themselves. Levitis figured that this complexity might lead to problems creating healthy progeny.

In new research published this week (Aug. 1, 2017) in the Proceedings of the Royal Society B, Levitis and his collaborators report that meiosis takes a heavy toll on the viability of offspring. And not just for humans. Creatures from geckos to garlic and cactuses to cockroaches pay a price to undergo sexual reproduction.

The work provides deeper context on the fundamental biological causes behind pregnancy loss, and suggests that the advantages of sexual reproduction must overcome the severe constraints imposed by meiosis.

"It's known that for humans, the primary cause of pregnancy loss is chromosomal abnormalities arising from meiosis," says Anne Pringle, a professor of botany at UW-Madison and another author of the research. "But what wasn't at all clear was whether meiosis is a leading cause of inviability not just in humans, but wherever it occurs."

The video will load shortly

To answer this question, Levitis compared the viability of offspring produced by three different kinds of reproduction. Sexual reproduction, where two players make a genetic contribution, always requires meiosis. On the other hand, asexual reproductionwhere the offspring are clones of their parentsusually uses the much simpler mitosis, a comparatively easy cloning of cells, no genetic reshuffling required. When asexual reproduction does use meiosis, it is even more complicated than sex.

In this three-way comparison, Levitis found that more complex reproduction resulted in lower offspring survival. For example, asexual lizards that use meiosis had lower viability than sexual lizards that also use meiosis because asexual meiosis was more complicated. Yet the organisms that used the simpler mitosis, like palm trees and damselflies, produced healthier offspring.

This pattern held true in 42 of 44 species. "When you get a result that consistent across such a wide range of organisms, it's suspicious," says Levitis. But even after a second look, the data checked out. Something about meiosis, seemingly its complexity, kills offspring.

"If you're making your tally sheet, all the pluses and minuses of sex, the fact that sex requires this deadly process is pretty clearly a disadvantage," says Levitis.

Regarding the evolution of sex, Levitis' findings suggest that the advantages of going through meiosis must be significant enough to balance that tally sheet. The reshuffling of genes between two parents during sex might provide even more of an advantage than previously thought.

The other takeaway, says Levitis, is that although it's easy to think that natural selection can solve every problemand that we might wish it had, such as for high rates of pregnancy losssometimes it comes up against fundamental constraints. Meiosis seems to be one of those insurmountable barriers.

Yet the tradeoff, offspring that are truly unique, with novel genetic combinations to face a challenging world, must be worth it.

Explore further: Researchers identify traffic cop mechanism for meiosis

More information: Is meiosis a fundamental cause of inviability among sexual and asexual plants and animals? Proceedings of the Royal Society B, rspb.royalsocietypublishing.or .1098/rspb.2017.0939

Researchers at NYU and the Whitehead Institute for Biomedical Research have identified the mechanism that plays "traffic cop" in meiosisthe process of cell division required in reproduction. Their findings, which appear ...

Researchers at New York University and the Whitehead Institute for Biomedical Research have identified the mechanism that plays "traffic cop" in meiosisthe process of cell division required in reproduction. Their findings, ...

Alternative splicing significantly expands the form and function of the genome of organisms with limited gene numbers and is especially important for several stages of mouse spermatogenesis.

Why is sex so popular among plants and animals, and why isn't asexual reproduction, or cloning, a more common reproductive strategy?

Where would we be without meiosis and recombination? For a start, none of us sexually reproducing organisms would be here, because that's how sperm and eggs are made. And when meiosis doesn't work properly, it can lead to ...

Geneticists have identified an enzyme which regulates the production of sperm and egg cells in human reproduction.

Researchers have been able to switch on and study the mechanism some bacteria use to inject toxins into their rivals.

All animals use a combination of senses to survive. But where the majority typically rely on one or two especially sensitive sensory systems, the oilbird excels by apparently having keen senses all-around.

An unspoken frustration for evolutionary biologists over the past 100 years, says Craig Albertson at the University of Massachusetts Amherst, is that genetics can only account for a small percentage of variation in the physical ...

Interpreting relationships between species and their environments is crucial to inform ecosystem-based management (EBM), a priority for NOAA Fisheries. EBM recognizes the diverse interactions within an ecosystemincluding ...

After Dan Levitis and his wife lost two pregnancies, before having their three children, he was drawn to investigate why pregnancy loss is so common, and whether other living beings face the same struggle his family did.

The first flower to appear along the path of plant evolution, during the time of the dinosaurs, was a hermaphrodite with petal-like organs arranged in concentric circles, researchers said Monday.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Read more:

Pregnancy loss and the evolution of sex are linked by cellular line dance - Phys.Org

) at a the laboratory of Santa Fe zoo in Medellin, Colombia. Many species of frogs, including the Golden Frog, the most venomous frog in the world, are in danger of extinction. Photograph: Raul Arboleda/AFP/Getty Images

Although Kermit the Frog has always struggled with body image, in evolutionary terms, the frog body plan is a rather successful one. With a short, stout body, protruding eyes and strong, flexible limbs with webbed feet, the world can be your swamp. The frog body plan has remained rather similar for almost 200m years, and with only limited tweaks in anatomy, frogs (Anura) have managed to occupy a range of different habitats, from muddy pools in Alaska to tree tops in the tropics. Currently, over 6700 species are known from all continents except Antarctica, which makes frogs one of the most diverse and species-rich groups of tetrapods. Never change a good thing. However, this limited variation in the frog body plan over time and space has made it difficult for biologists to reconstruct the evolutionary history of frogs and to sort out who is related to who.

Frogs are amphibians, and the oldest member of the frog lineage the stem-frog Triadobatrachus massinoti which lived during the Early Triassic (~250m years ago) in what is now Madagascar still retained primitive features, such as a tail and the likely inability to jump, that distinguish it from modern frogs. By the Early Cretaceous (131-120m years ago), the first members of the modern frogs have evolved, such as the three-dimensionally preserved Liaobatrachus zhaoi from the Yixian Formation in China (Dong et al., 2013).

The rise of molecular techniques enabled scientists to use DNA instead of morphology to try to unravel the frog family tree. Initial studies focused only on a limited number of genes, and as a result, age estimates for certain groups of frogs varied wildly. Moreover, these studied did little to understand relationships within frog groups, particularly for those groups that contain massive numbers of species, such as the Hyloidea which includes the glass frogs and poison-dart frogs.

A new study by Yan-Lie Feng and colleagues from Sun Yat-Sen University in Guangzhou, China, set out to change this by using an extensive molecular dataset that included 95 different genes from 156 species of frogs. Their dataset represents a major leap compared to previous studies, and has resulted in the best supported timescale of frog evolution thusfar.

By using such a large number of genes and species, this new analysis indicates that the major frog groups are younger than previously thought. The last common ancestor of all living frogs (crown-group Anura) is estimated to have lived during the Upper Triassic at 210m years ago. This is in contrast to previous studies that placed the last common ancestor much further back in time at around 250 million years.

When plotting the diversification of frogs on a geological time scale, it becomes clear that diversification events in frogs coincide with break-ups of major prehistoric landmasses. The first split within Anura is that between the Neobatrachia and other anurans. This split occurs at ~ 180 Mya (Middle Jurassic), at around the same time as the breakup of Pangaea into the two supercontinents Laurasia (northern hemisphere) and Gondwana (southern hemisphere). A second break up occurred at around 135m years ago, when two major lineages of Neobatrachia split into Proceola, containing the superfamily Hyloidea, and Diplasiocoela including the Ranoidea. This split coincides with the separation of South America and Africa and the spreading of the South Atlantic Ocean sea floor in the Early Cretaceous.

Interestingly, three major lineages of frogs, the Hyloidea, Microhylidae and Natatanura, have a near-synchronous origin at around 66m years ago. That estimate overlaps with a major extinction event, the Cretaceous Tertiary (K/T) mass extinction, during which two-thirds of life on earth, including the non-avian dinosaurs, marine reptiles and pterosaurs, went extinct.

Although there is little fossil evidence to show how the K/T mass extinction impacted frogs, it is not unlikely that a number of frog species went the way of the dinosaurs. However, when researchers looked at the rate at which species originated during that time period, the analysis indicated that there was a surge in frog diversification immediately following the K/T boundary. Quite ribbiting, perhaps, is the fact that 88% of current frog species originated in this relatively short time period after the K/T mass extinctions.

Mass extinctions leave behind a wasteland of empty ecological real estate. Species that survive can take advantage of this empty ecological space, and as different organisms invade different niches, they adapt and diversify. This is why mass extinction events are often followed by periods of rapid adaptive radiation and speciation.

The demise of non-avian dinosaurs and many other groups at the end of the Cretaceous triggered explosive radiations of mammals (Alroy, 1999) and birds (Ksepka et al., 2017). This new study on frogs shows that the aftermath of the K/T mass extinction may have provided new ecological opportunities for amphibians as well. Particularly, the increase in forest habitats after the massive loss of vegetation that happened at the K/T boundary is thought to have played a major role in enabling adaptive radiations for arboreal taxa. Truly arboreal species of frogs are limited to groups that originated after the K/T boundary, demonstrating how mass extinctions in the past have shaped the current diversity of frogs. However, as past performance is no guarantee for future success, it remains to be seen how frogs will do in the next round of mass extinction.

References

Alroy, J. 1999. The fossil record of North American mammals: evidence for a Paleocene evolutionary radiation. Systematic Biology 48 (1).

Dong, L., Roek, Z., Wang, Y., and Jones, M.E.H. 2013. Anurans from the Lower Cretaceous Jehol Group of Western Liaoning, China. PLoS ONE 8 (12)

Feng, Y-J., Blackburn; D.C., Liang, D., Hillis, D.M., Wake, D.B., Cannatella, D.C., and Zhang, P. 2017. Phylogenomics reveals rapid, simultaneous diversification of three major clades of Gondwanan frogs at the Cretaceous Paleogene boundary. PNAS 114(29)

Ksepka D.T., Stidham, T.A., and Williamson, T.E. 2017. Early Paleocene landbird supports rapid phylogenetic and morphological diversification of crown birds after the KPg mass extinction. PNAS 114 (30)

See the rest here:

Jump for joy: researchers make huge leap in understanding frog evolution - The Guardian