Saturday, March 8, 2008
Friday, March 7, 2008
Thursday, March 6, 2008
Why Cells That Become Sperm And Ova Can't Copy Their Own Genes
Researchers in Kobe, Japan, and Montreal, Canada, have uncovered a previously unknown mechanism which causes embryonic germ cells -- which later develop into sperm or ova -- to go through a period of "transcriptional silence," during which information from the cell's DNA cannot be copied. Without this important phase, unique to cells of this type, an organism produces sterile offspring.
The study was conducted by a team led by Dr. Akira Nakamura at the RIKEN Center for Developmental Biology (CDB) in Kobe and by Dr. Paul Lasko, Chair of McGill University's Department of Biology. Their results were published in January, 2008, in the journal Nature.
"A fundamental characteristic of embryonic germ cells in all organisms is that they don't transcribe their own genes for a certain time during embryonic development," Dr. Lasko explained. "They are transcriptionally silent; that's what makes them special. It's not fully understood why this is the case, but if that silencing doesn't happen, then the germ cells don't work. They don't migrate correctly and they don't make their way into the gonads."
Dr. Nakamura was a post-doctoral fellow in Dr. Lasko's lab in the mid-1990s when they co-discovered the Polar Granule Component (PGC) gene in drosophila, commonly known as the fruit fly. If the mother fly lacks PGC, her offspring will be unable to produce germ cells. Initially, Dr. Lasko said, they discovered that the PGC gene produced an RNA, but they did not believe it produced any proteins. Using current technology, Dr. Nakamura discovered that PGC does indeed produce a protein which regulates Transcription Elongation Factor B (TEF-B), the genetic machinery that expresses proteins.
"It's a very small, 71-amino acid protein," Dr. Lasko explained. "The average length of a protein is about 400 to 500 amino acids, so this is extremely small. Back when we did the initial research, there weren't very many genes known that encoded such a short protein. The significance of this is that Nakamura has shown that this little protein seems to be the key regulator that keeps gene expression shut off in germ cells."
Mutant fruit flies without the ability to produce the protein produce sterile offspring which produce no sperm or eggs.
"What the study argues is that this regulation of TEF-B might be very important for germ cell development in a variety of organisms. That's something people will want to look at in mammals," Dr. Lasko said.
The study was conducted by a team led by Dr. Akira Nakamura at the RIKEN Center for Developmental Biology (CDB) in Kobe and by Dr. Paul Lasko, Chair of McGill University's Department of Biology. Their results were published in January, 2008, in the journal Nature.
"A fundamental characteristic of embryonic germ cells in all organisms is that they don't transcribe their own genes for a certain time during embryonic development," Dr. Lasko explained. "They are transcriptionally silent; that's what makes them special. It's not fully understood why this is the case, but if that silencing doesn't happen, then the germ cells don't work. They don't migrate correctly and they don't make their way into the gonads."
Dr. Nakamura was a post-doctoral fellow in Dr. Lasko's lab in the mid-1990s when they co-discovered the Polar Granule Component (PGC) gene in drosophila, commonly known as the fruit fly. If the mother fly lacks PGC, her offspring will be unable to produce germ cells. Initially, Dr. Lasko said, they discovered that the PGC gene produced an RNA, but they did not believe it produced any proteins. Using current technology, Dr. Nakamura discovered that PGC does indeed produce a protein which regulates Transcription Elongation Factor B (TEF-B), the genetic machinery that expresses proteins.
"It's a very small, 71-amino acid protein," Dr. Lasko explained. "The average length of a protein is about 400 to 500 amino acids, so this is extremely small. Back when we did the initial research, there weren't very many genes known that encoded such a short protein. The significance of this is that Nakamura has shown that this little protein seems to be the key regulator that keeps gene expression shut off in germ cells."
Mutant fruit flies without the ability to produce the protein produce sterile offspring which produce no sperm or eggs.
"What the study argues is that this regulation of TEF-B might be very important for germ cell development in a variety of organisms. That's something people will want to look at in mammals," Dr. Lasko said.
Wednesday, March 5, 2008
New York Times Examines Potential Causes Of Increased Fertility Rate In U.S.A.
The New York Times on Friday examined potential reasons for the increased fertility rate in the U.S., including changes in the real estate market. According to recent data from CDC's National Center for Health Statistics, the total fertility rate -- the number of children a woman will have in her lifetime -- increased to 2.1 children per woman in 2006, the highest number since 1961. The rise reflects increases in birth rates among women in all parts of the country and almost every demographic group except girls under age 15, which was the only decline, the Times reports.
The report found that the fertility rate among Hispanic women in 2006 was the highest with 2.96 children per woman, compared with 2.11 for non-Hispanic black women and 1.86 for non-Hispanic whites. General birth rates were highest in Republican "strongholds" -- Utah had the highest rate followed by Arizona, Idaho and Texas -- the Times reports. Birth rates were lowest in states won by Sen. John Kerry (D-Mass.) in the 2004 presidential election -- Vermont had the lowest followed by New Hampshire, Maine, Rhode Island, New York, New Jersey, Connecticut and Massachusetts -- the Times noted. The report did not include information on religion or socioeconomic status, but researchers have associated religious affiliation and observance with increased fertility rates. A 2007 survey by the Pew Research Center found that 79% of evangelicals said they had children, compared with 73% of nonevangelical Protestants and 62 percent of those who described themselves as secular.
Demographers say it is too early to determine if the increase is a trend or to determine its causes, which might include changes in immigration, the economy and the availability of abortion. Stephanie Ventura, chief of NCHS' reproductive statistics branch, said the increase "could turn around on a dime" but added it was unusual that birth rates in 2006 increased for both teenagers and older women. In the past, a strong economy "contributed to a decline in the teenage birth rate because they saw they could get good jobs, so they put off childbirth," Ventura said, adding, "For older people, a good economy makes them say, 'We can afford to have another child.'"
Robert Engelman -- vice president for programs at the Worldwatch Institute and author of "More: Population, Nature and What Women Want" -- said the availability of housing in the U.S. might be linked to the increased fertility rates. "One reason there are so few children in Italy is that housing is so hard to come by," Engelman said, adding, "Houses are bigger in the U.S. and generally more available. That may help explain why" U.S. residents have more children.
Several population specialists said that housing is one influence on fertility and that it is difficult to ignore other variables, including income or optimism. "If you lower the cost of housing, you're going to lower the cost of raising a child," Seth Sanders, director of University of Maryland's Maryland Population Research Center, said, adding, "But if you look at how much it costs to raise a child, only one-third of the cost is housing. So my guess is that the impact is not very large."
Morris Davis, assistant professor of real estate and urban land economics at the University of Wisconsin-Madison School of Business, said the link between housing and fertility is "something a bunch of us have been thinking about," adding, "If you reduce down-payment constraints, more people can buy homes or buy bigger homes. Does that encourage them to have more kids? I would say nobody knows".
The report found that the fertility rate among Hispanic women in 2006 was the highest with 2.96 children per woman, compared with 2.11 for non-Hispanic black women and 1.86 for non-Hispanic whites. General birth rates were highest in Republican "strongholds" -- Utah had the highest rate followed by Arizona, Idaho and Texas -- the Times reports. Birth rates were lowest in states won by Sen. John Kerry (D-Mass.) in the 2004 presidential election -- Vermont had the lowest followed by New Hampshire, Maine, Rhode Island, New York, New Jersey, Connecticut and Massachusetts -- the Times noted. The report did not include information on religion or socioeconomic status, but researchers have associated religious affiliation and observance with increased fertility rates. A 2007 survey by the Pew Research Center found that 79% of evangelicals said they had children, compared with 73% of nonevangelical Protestants and 62 percent of those who described themselves as secular.
Demographers say it is too early to determine if the increase is a trend or to determine its causes, which might include changes in immigration, the economy and the availability of abortion. Stephanie Ventura, chief of NCHS' reproductive statistics branch, said the increase "could turn around on a dime" but added it was unusual that birth rates in 2006 increased for both teenagers and older women. In the past, a strong economy "contributed to a decline in the teenage birth rate because they saw they could get good jobs, so they put off childbirth," Ventura said, adding, "For older people, a good economy makes them say, 'We can afford to have another child.'"
Robert Engelman -- vice president for programs at the Worldwatch Institute and author of "More: Population, Nature and What Women Want" -- said the availability of housing in the U.S. might be linked to the increased fertility rates. "One reason there are so few children in Italy is that housing is so hard to come by," Engelman said, adding, "Houses are bigger in the U.S. and generally more available. That may help explain why" U.S. residents have more children.
Several population specialists said that housing is one influence on fertility and that it is difficult to ignore other variables, including income or optimism. "If you lower the cost of housing, you're going to lower the cost of raising a child," Seth Sanders, director of University of Maryland's Maryland Population Research Center, said, adding, "But if you look at how much it costs to raise a child, only one-third of the cost is housing. So my guess is that the impact is not very large."
Morris Davis, assistant professor of real estate and urban land economics at the University of Wisconsin-Madison School of Business, said the link between housing and fertility is "something a bunch of us have been thinking about," adding, "If you reduce down-payment constraints, more people can buy homes or buy bigger homes. Does that encourage them to have more kids? I would say nobody knows".
Tuesday, March 4, 2008
Closer Kinship Linked With Reproductive Success
In a paper published 7 Feb 2008 deCODE scientists establish a substantial and consistent positive correlation between the kinship of couples and the number of children and grandchildren they have. The study, which analyzes more than 200 years of deCODE's comprehensive genalogical data on the population of Iceland, shows that couples related at the level of third cousins have the greatest number of offspring. For example, for women born between 1800 and 1824, those with a mate related at the level of a third cousin had an average of 4.04 children and 9.17 grandchildren, while those related to their mates as eighth cousins or more distantly had 3.34 children and 7.31 grandchildren. For women born in the period 1925-1949 with mates related at the degree of third cousins, the average number of children and grandchildren were 3.27 and 6.64, compared to 2.45 and 4.86 for those with mates who were eighth cousins or more distantly related.
The findings hold for every 25-year interval studied, beginning with those born in the year 1800 up to the present day. Because of the strength and consistency of the association, even between couples with very subtle differences in kinship, the authors conclude that the effect very likely has a biological basis, one which has yet to be elucidated. The paper, 'An association between the kinship and fertility of human couples,' is published online in Science magazine at www.sciencemag.org.
This study provides the most comprehensive answer yet to the longstanding question of how kinship affects fertility in humans. Previous studies in other parts of the world have suggested that the two phenomena are positively correlated, though confounding variables, such as the impact of socioeconomic status on the size of families or age at marriage, have made the results difficult to interpret. The analysis of such a long-term series of data from Iceland effectively eliminates these variables by encompassing an entire population which has historically been highly homogeneous both culturally and economically. Moreover, the results are strikingly consistent from eras in which Iceland was a predominantly poor and rural country, to the present-day era of a highly urbanized society with one of the highest standards of living in the world.
The authors note that the findings are somewhat counterintuitive from an evolutionary perspective because closely-related parents have a higher probability of having offspring homozygous for deleterious recessive mutations, although closer parental kinship can also decrease the likelihood of immunological incompatibility between mother and offspring, for example in rhesus factor blood type. Perhaps most importantly, today's findings also suggest that the recent and dramatic demographic shift experienced in Iceland - from a rural society to a highly urbanized one - may serve to slow population growth, as individuals are exposed to a much broader range of distantly related potential mates. If so, this could be of relevance to slowing population growth in the many other - and much more populous - societies around the world undergoing transition from closely-knit rural societies to more urbanized ones. Indeed, the UN estimates that in the 2007-2008 period the majority of the world's population will, for the first time in human history, live in town and cities.
The findings hold for every 25-year interval studied, beginning with those born in the year 1800 up to the present day. Because of the strength and consistency of the association, even between couples with very subtle differences in kinship, the authors conclude that the effect very likely has a biological basis, one which has yet to be elucidated. The paper, 'An association between the kinship and fertility of human couples,' is published online in Science magazine at www.sciencemag.org.
This study provides the most comprehensive answer yet to the longstanding question of how kinship affects fertility in humans. Previous studies in other parts of the world have suggested that the two phenomena are positively correlated, though confounding variables, such as the impact of socioeconomic status on the size of families or age at marriage, have made the results difficult to interpret. The analysis of such a long-term series of data from Iceland effectively eliminates these variables by encompassing an entire population which has historically been highly homogeneous both culturally and economically. Moreover, the results are strikingly consistent from eras in which Iceland was a predominantly poor and rural country, to the present-day era of a highly urbanized society with one of the highest standards of living in the world.
The authors note that the findings are somewhat counterintuitive from an evolutionary perspective because closely-related parents have a higher probability of having offspring homozygous for deleterious recessive mutations, although closer parental kinship can also decrease the likelihood of immunological incompatibility between mother and offspring, for example in rhesus factor blood type. Perhaps most importantly, today's findings also suggest that the recent and dramatic demographic shift experienced in Iceland - from a rural society to a highly urbanized one - may serve to slow population growth, as individuals are exposed to a much broader range of distantly related potential mates. If so, this could be of relevance to slowing population growth in the many other - and much more populous - societies around the world undergoing transition from closely-knit rural societies to more urbanized ones. Indeed, the UN estimates that in the 2007-2008 period the majority of the world's population will, for the first time in human history, live in town and cities.
Monday, March 3, 2008
Sunday, March 2, 2008
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