Tracing the Origin & Evolution of Bird Reproduction in Dinosaurs

What really did come first--the chicken or the egg? Birds' reproductive biology is dramatically different from that of any other living vertebrates, and ornithologists and paleontologists have long wondered how and when the unique features of bird reproduction originated. A new Review in The Auk: Ornithological Advances examines answers from three sources--modern birds, fossils of primitive birds, and fossils of the dinosaurs from which birds are descended--to shed new light on the subject.

All modern birds share certain reproductive features, such as a single functional ovary and the practice of incubating their eggs through direct contact. Analysis of the bird family tree also suggests that early birds built simple, open nests on the ground and that their young were "precocial," meaning they were well-developed and almost ready to fend for themselves when they hatched. Those dinosaurs close to the ancestry of birds shared some of these traits, but they had two functional reproductive tracts, and their eggs were smaller relative to their body size and more elongated than those of modern birds.

Fossils of primitive birds and eggs from the Mesozoic era place them midway between their dinosaur ancestors and their modern descendants, with eggs between those of pre-avian dinosaurs and modern birds in term of size and shape. In this way, David Varricchio and Frankie Jackson of the Montana State University are able to trace the evolution of bird reproduction through a series of distinct stages, from pre-avian dinosaurs to the birds of today.

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Bdelloid rotifers Exchange DNA Like Bacteria do, a First for Metazoans (animals)

Bdelloid rotifers were believed to have persisted and diversified in the absence of sex. Two papers now show they exchange genes with each other, via horizontal gene transfers as known in bacteria and/or via other forms of non-canonical sex.

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Men Close to Obsolete? Scientists Produce Sperm From Stem Cells

Researchers in China say that they have discovered a way to make rudimentary mouse sperm in a dish, and used them to produce offspring.

If the claim stands up to scrutiny, it could point the way to making human sperm in the lab for fertility treatments. But some scientists are not convinced by the report, which is published today inCell Stem Cell.

“The results are super-exciting and important,” says Jacob Hanna, a stem-cell scientist at the Weizmann Institute of Science in Rehovot, Israel. But Takashi Shinohara, a reproductive biologist at Kyoto University in Japan, is among researchers who have doubts about the work: he notes that scientists have struggled to replicate several previous claims that sperm can be made in a dish.

In 2011, molecular biologists led by Mitinori Saitou at Kyoto University reported that they had managed to recreate the first stages of sperm development in a dish. They coaxed mouse embryonic stem cells to become cells that resembled primordial germ cells (PGCs)—an important stage in the development of both eggs and sperm.

Saitou's team then implanted the artificial PGCs into a mouse: when implanted in testes, they grew to become sperm; in ovaries, they matured into eggs.

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Using a Virus to Permanently Sterilize Animals

Caltech biologists have developed a nonsurgical method to deliver long-term contraception to both male and female animals with a single shot. The technique--so far used only in mice--holds promise as an alternative to spaying and neutering feral animals.

The approach was developed in the lab of Bruce Hay, professor of biology and biological engineering at Caltech, and is described in the October 5 issue of Current Biology. The lead author on the paper is postdoctoral scholar Juan Li.

Hay's team was inspired by work conducted in recent years by David Baltimore and others showing that an adeno-associated virus (AAV)--a small, harmless virus that is unable to replicate on its own, that has been useful in gene-therapy trials--can be used to deliver sequences of DNA to muscle cells, causing them to produce specific antibodies that are known to fight infectious diseases, such as HIV, malaria, and hepatitis C.

Li and her colleagues thought the same approach could be used to produce infertility. They used an AAV to deliver a gene that directs muscle cells to produce an antibody that neutralizes gonadotropin-releasing hormone (GnRH) in mice. GnRH is what the researchers refer to as a "master regulator of reproduction" in vertebrates--it stimulates the release of two hormones from the pituitary that promote the formation of eggs, sperm, and sex steroids. Without it, an animal is rendered infertile.

In the past, other teams have tried neutralizing GnRH through vaccination. However, the loss of fertility that was seen in those cases was often temporary. In the new study, Hay and his colleagues saw that the mice--both male and female--were unable to conceive after about two months, and the majority remained infertile for the remainder of their lives.

"Inhibiting GnRH is an ideal way to inhibit fertility and behaviors caused by sex steroids, such as aggression and territoriality," says Hay. He notes that in the study, his team also shows that female mice can be rendered infertile using a different antibody that targets a binding site for sperm on the egg. "This target is ideal when you want to inhibit fertility but want to leave the individual otherwise completely normal in terms of reproductive behaviors and hormonal cycling."

Hay's team has dubbed the new approach "vectored contraception" and says that there are many other proteins that are thought to be important for reproduction that might also be targeted by this technique.

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I am not at all afraid of whom might get this and the greater implications of this technology.  Nothing could possibly go wrong here!

Pterosaurs had two Ovaries, Seem to Lack Medullary Bone

Eggshell and Histology Provide Insight on the Life History of a Pterosaur with Two Functional Ovaries

Authors:

Wang et al

Abstract:

The counterpart of a previously described non-pterodactyloid pterosaur with an egg revealed the presence of a second egg inside the body cavity of this gravid female. It clearly shows that pterosaurs had two functional oviducts and demonstrates that the reduction of one oviduct was not a prerequisite for developing powered flight, at least in this group. Compositional analysis of one egg suggests the lack of a hard external layer of calcium carbonate. Histological sections of one femur lack medullary bone and further demonstrate that this pterosaur reached reproductive maturity before skeletal maturity. This study shows that pterosaurs laid eggs even smaller than previously thought and had a reproductive strategy more similar to basal reptiles than to birds. Whether pterosaurs were highly precocial or needed parental care is still open to debate.

Scientists Produce Germ-line Cells From Human Skin

Researchers from Cambridge University and Israel’s Weizmann Institute of Science are claiming a stem cell research breakthrough that would allow a baby to be created from the skin cells from two adults, no matter their gender. This potentially allows for infertile couples to have their own children without resorting to sperm or egg donors, and may provide the means for same sex couples to produce their own babies.

Previously only successful in experiments on mice, the new research has been conducted on human cells for the first time. In this study, the researchers paired stem cell lines from embryos with the skin of a range of different adults, with the resultant cells compared to aborted fetuses to determine an identical match.

Techniques devised to create same-sex offspring are not new. Some experiments involve the manipulation of fibroblasts in mice resulting in offspring with the genetic traits of multiple male mice, whilst others have used bone marrow stem cells extracted from males to trigger spermatogonia.

However, in this latest research, stem cells and adult human skin have been combined for the first time to create an entire new germ-cell line (that is, cells that will become embryos). Derived from ten different donor sources, the new germ-cell lines were created from 10 different donor sources – five embryos and five adults.

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Dads: You're the Reason You're kid is a Mutant Freak (ahem)

The offspring of chimpanzees inherit 90% of new mutations from their father, and just 10% from their mother, a finding which demonstrates how mutation differs between humans and our closest living relatives, and emphasises the importance of father's age on evolution.

Published today in Science, researchers from the Wellcome Trust Centre for Human Genetics and the Biomedical Primate research Centre in the Netherlands looked at whether, in chimpanzees, there was a heightened risk of fathers passing on mutations to their children compared to humans.

In humans, each individual inherits, on average, about 70 new mutations from their parents. However, this number is influenced by paternal age such that older fathers tend to result in more mutations – in humans each extra year of age results in two extra mutations.

Mutation risk is linked to father's age because the sperm lineage in males keeps dividing, while females have all the eggs they are ever going to produce present at birth. Paternal age is an established risk factor in a number of disorders including schizophrenia and autism.

The study found that the number of new mutations inherited by chimpanzees from their parents is, on average, very similar to that in humans, but that the effect of the father's age is much stronger – each additional year of father's age results in three extra mutations.

The results suggest that sexual selection can influence the rate of evolution through its effect on the male mutation rate.

Professor Gil McVean, from the Wellcome Trust Centre for Human Genetics at the University of Oxford said: "In humans, a father's age is known to affect how many new mutations he passes on to his children, and is also an established risk factor in a number of mental health disorders.

"This study finds that in chimpanzees the father's age has a much stronger effect on mutation rate – about one and a half times that in humans. As a consequence, a greater fraction of new mutations enter the population through males, around 90 per cent, compared to humans, where fathers account for 75 per cent of new mutations."

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Basal Birds Were Reproductively Modern

As winged dinosaurs underwent a series of evolutionary changes during the transition into Aves, or birds, one pivotal transformation was the appearance of a single-ovary reproductive system. "The most widely accepted hypothesis for the presence of a single functional ovary in living birds is that the right ovary … was lost to reduce body mass in gravid females during flight," report a team of Chinese scientists who are adding new details to the mosaic of understanding how terrestrial dinosaurs gave rise to birds and powered flight.

These scientists, led by the director of the prestigious Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, posit that this adaptation of the reproductive system likely occurred "gradually during the evolution of dinosaurs and basal birds," yet add that until recently, the fossil record provided only scattered evidence on the timeline of these phylogenetic changes.

But the discovery of a series of fossils that include evidence of single ovaries in the basal bird Jeholornis and in an array of more derived enantiornithine birds from the Cretaceous, in what is now eastern China, is helping them pinpoint the timing of the pared-down reproductive system of avian dinosaurs.

Members of Aves, which includes the common ancestor of the 150 million-year-old Archaeopteryx and all living birds, are unique among amniotes in terms of featuring a single-ovary system, notes Zhonghe Zhou, director of the IVPP and lead author of the study "Ovarian follicles shed new light on dinosaur reproduction during the transition towards birds."

One feathered dinosaur considered closely related to birds, the oviraptorosaurian maniraptoran theropod, had two functional ovaries, according to Zhou and co-authors of the paper.

In contrast, they add, the fossil of the primitive bird Jeholornis that was recently uncovered in the northeastern Chinese province of Liaoning featured the lighter-weight single ovary system. "Jeholornis," they explain, "with its long dinosaurian boney tail, is only slightly more derived than Archaeopteryx, indicating that even the most basal birds were already modern in this aspect."

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Semelparity "Explained" in Marsupial Mammals

Sperm competition drives the evolution of suicidal reproduction in mammals

Authors:

Diana O. Fisher, Christopher R. Dickman, Menna E. Jones and Simon P. Blomberg

Abstract:

Suicidal reproduction (semelparity) has evolved in only four genera of mammals. In these insectivorous marsupials, all males die after mating, when failure of the corticosteroid feedback mechanism elevates stress hormone levels during the mating season and causes lethal immune system collapse (die-off). We quantitatively test and resolve the evolutionary causes of this surprising and extreme life history strategy. We show that as marsupial predators in Australia, South America, and Papua New Guinea diversified into higher latitudes, seasonal predictability in abundance of their arthropod prey increased in multiple habitats. More-predictable prey peaks were associated with shorter annual breeding seasons, consistent with the suggestion that females accrue fitness benefits by timing peak energy demands of reproduction to coincide with maximum food abundance. We demonstrate that short mating seasons intensified reproductive competition between males, increasing male energy investment in copulations and reducing male postmating survival. However, predictability of annual prey cycles alone does not explain suicidal reproduction, because unlike insect abundance, peak ovulation dates in semelparous species are often synchronized to the day among years, triggered by a species-specific rate of change of photoperiod. Among species with low postmating male survival, we show that those with suicidal reproduction have shorter mating seasons and larger testes relative to body size. This indicates that lethal effort is adaptive in males because females escalate sperm competition by further shortening and synchronizing the annual mating period and mating promiscuously. We conclude that precopulatory sexual selection by females favored the evolution of suicidal reproduction in mammals.

Why No Turtles Are Viviparous

Limited Oxygen Availability In Utero May Constrain the Evolution of Live Birth in Reptiles

Authors:

1. Anthony R. Rafferty (a)

2. Roger G. Evans (b)

3. T. Franciscus Scheelings (c)

4. Richard D. Reina (a)

Affiliations:

a. Australian Centre for Biodiversity, School of Biological Sciences, Monash University, Clayton, Victoria 3800, Australia

b. Department of Physiology, Monash University, Clayton, Victoria 3800, Australia

c. Australian Wildlife Health Centre, Healesville Sanctuary, Healesville, Victoria 3777, Australia

Abstract:

Although viviparity (live birth) has evolved from oviparity (egg laying) at least 140 times in vertebrates, nearly 120 of these independent events occurred within a single reptile taxon. Surprisingly, only squamate reptiles (lizards and snakes) are capable of facilitating embryonic development to increasingly advanced stages inside the mother during extended periods of oviducal egg retention. Viviparity has never evolved in turtle lineages, presumably because embryos enter and remain in an arrested state until after eggs are laid, regardless of the duration of egg retention. Until now, the limiting factor that initiates and maintains developmental arrest has remained elusive. Here, we show that oviducal hypoxia arrests embryonic development. We demonstrate that hypoxia can maintain developmental arrest after oviposition and that subsequent exposure of arrested embryos to normoxia triggers resumption of their development. We discovered remarkably low oxygen partial pressure in the oviducts of gravid turtles and found that secretions produced by the oviduct retard oxygen diffusion. Our results suggest that an extremely hypoxic environment in the oviduct arrests embryonic development and may constrain the evolution of viviparity in turtles, with the reduced diffusive capacity of oviducal secretions possibly creating or contributing to this hypoxia. We anticipate that these findings will allow us to better understand the mechanisms underlying the evolutionary transition between reproductive modes.

Sperm From Stem Cells

Human sperm have been created using embryonic stem cells for the first time in a scientific development which will lead researchers to a better understanding of the causes of infertility.

Researchers led by Professor Karim Nayernia at Newcastle University and the NorthEast England Stem Cell Institute (NESCI) have developed a new technique which has made the creation of human sperm possible in the laboratory.

The work is published today (8th July 2009) in the academic journal Stem Cells and Development.

[...]

In the technique developed at Newcastle, stem cells with XY chromosomes (male) were developed into germline stem cells which were then prompted to complete meiosis - cell division with halving of the chromosome set. These were shown to produce fully mature, sperm called scientifically, In Vitro Derived sperm (IVD sperm).

In contrast, stem cells with XX chromosomes (female) were prompted to form early stage sperm, spermatagonia, but did not progress further. This demonstrates to researchers that the genes on a Y chromosome are essential for meiosis and for sperm maturation.

Can't quite do away with us yet ladies.