Inbreeding in the Darwin dynasty?

Darwin and his wife were first cousins

Charles Darwin married his first cousin, Emma Wedgwood, and his own mother was the product of a marriage between third cousins. Given his insights into the relationship among variation, nature’s choices, and adaptation and his observations of weakening in inbred plants, it is no surprise that Darwin worried about his own family’s consanguinity. Did the inbreeding in the Darwin/Wedgwood families show up in his children?

Is marrying your first cousin really so bad?

Had the Darwin/Wedgwoods only engaged in the first-cousin marriage between Charles and Emma, the outcome would likely not have been serious. A 2002 study reported by the National Society of Genetic Counselors found that having first cousins as parents raises the risk of having a significant genetic defect from 3-4% up to about 4-7%. The group concluded that first cousins planning to reproduce require no more intense genetic counseling than unrelated couples.

Consistent consanguinity, on the other hand

But that study didn’t address serial consanguinity of the kind seen in some European royal houses or in the Darwin/Wedgwood families. And a new analysis reported in BioScience avers that the Darwin offspring did experience the repercussions of such inbreeding. Applying an inbreeding coefficient to calculate whether childhood mortality in the Darwin/Wedgwood family across several generations was related to inbreeding, the authors indeed found an association.

Three of the Darwins’ ten children died at age 10 or younger, one of tuberculosis, one of scarlet fever, and one of an unidentified disease. Studies suggest an association between childhood mortality from bacterial infection and consanguinity, and the Darwin family seems to bear that out. In addition, three of the Darwin children who did live to adulthood experienced lengthy marriages without any children, and such infertility may be another manifestation of homozygous states that interfere with reproduction. A photograph of the youngest Darwin child, Charles, who died in toddlerhood, suggests that the baby had some congenital disorder, although the nature of it remains unclear. Emma Darwin was 48 years old when she gave birth to Charles, so Down Syndrome is one likely explanation.

Successful Darwins

In spite of some of the sad facts of the Darwin family story, a few of his children experienced successes of different kinds. Three of his sons were members of the Royal Society, a long-time Darwin family tradition that skipped over the most famous member of the tribe, Charles himself. And Darwin by any measure of fitness did pretty well: in spite of the loss of three children and the infertility of three children, he nevertheless had several grandchildren.

Did Darwin himself suffer from the effects of inbreeding?

Charles Darwin experienced a variety of chronic health conditions, but they do not necessarily seem to have been related to his family’s consanguineous status. Several theories abound to explain his symptoms, which included digestive and skin problems, but no one knows for certain what afflicted the great naturalist. One of the foremost hypotheses is that he had Chagas disease, occurring after a bug bite on one of his voyages transferred an infectious protozoan that may have permanently damaged the scientist’s gut. Stress seems to have exacerbated the problem, whatever its etiology.

The mysterious reproductive life of the giant panda

Photo credit: Mehgan Murphy, Smithsonian’s National Zoo

National Zoo’s giant panda had pseudopregnancy

National Zoo officials announced today that Mei Xiang (link has Panda Cam!), who had been monitored for several months for pregnancy, was not pregnant after all. Instead, she was experiencing a common feature of panda endocrinology, the pseudopregnancy.

Panda pseudopregnancy a common event

How could officials not be sure for months about whether or not the pregnancy was real? Panda pseudopregnancy so perfectly mimics an actual pregnancy that even hormone levels follow those of a real gestation. Staff had been monitoring her by ultrasound and blood testing, and even though ultrasound had yet to show a viable fetus, whether the pregnancy was real or pseudo was not confirmed until the hormones wrote the final chapter.

Pseudopregnancy hormones like pregnancy hormones

Late this month, Mei Xiang showed a drop in progesterone hormone. When hormone levels hit baseline in a possibly pregnant panda, one of two things can happen: a birth, or confirmation of pseudopregnancy. The progesterone decline set the clock on a 24-hour watch to see if Mei Xiang would bear a cub. She didn’t.

Ovulation once a year!

Giant pandas ovulate only once a year. Regardless of whether conception occurs, the female panda will appear pregnant, behave as though she is pregnant, and register the hormone patterns of pregnancy. If conception does not occur in that one annual opportunity, a female panda will almost always enter into a pseudopregnant state. Mei Xiang has done that five times. She’s also experienced a genuine pregnancy, bearing a cub in 2005 that now lives in China as part of a panda breeding program.

Panda soon to be back for public viewing

Mei Xiang has been sequestered during her pseudopregnancy, but her habitat at the zoo will now open again for public viewing. During her pseudopregnancy, her behaviors included reduced activity and appetite. These are now both expected to increase.

For your consideration

Pandas have some unusual life history strategies, including being food specialists and often accidentally suffocating their offspring. And, it appears that many ovulations result in pseudopregnancy. What might be an explanation for why pandas are so prone to entering a pseudopregnant state if conception does not occur? Could the behaviors that accompany the pseudopregnancy have anything to do with it?

In pandas, the hormones of a pseudopregnancy are similar to those of a real pregnancy. What pathways underlie the female’s production of these hormones of pseudopregnancy?

Women can also experience pseudopregnancy, sometimes referred to as “hysterical pregnancy.” It can even involve abdominal distention and in some cases, hormonal changes. What are some of the physiological underpinnings of a pseudopregnancy in women?

Finally, dogs and mice are also known for having pseudopregnancies. Do you think the pressures that result in these pseudopregnancies are similar to those that result in a false pregnancy in the panda? Why or why not?

Can rain make buffalos have boys?

African buffalo shift sex ratios with rain

African buffalos (Syncerus caffer) have more males during the rainy season in Kruger National Park, and it’s not just a random accident of fate. Researchers have found that specific sequences on the Y chromosome are correlated with seasonal differences in birth sex ratios in the buffalo population.

X sperm vs. Y sperm

Does that mean that rain somehow makes buffalos have more boys? Not directly. Instead, it may come down to a DNA-level battle royale involving the Y chromosome. Sometimes, sperm carrying the Y win the race to the egg, while at other times, X-carrying sperm are the victors. These times correlate with higher frequencies of certain sequences, or haplotypes, of the Y chromosome occurring in the population, with one sequence being much more common during the rainy season, when more males are born.

Selfish genes gone rogue

The investigation suggested the existence of a suppressor of Y chromosome success acting during the dry season, when females birthed more females, and a distorter in favor of Y chromosome success in the rainy season, when more males are born. The distorter may shift meiosis in favor of the Y-carrying sperm or disrupt survival of X-carrying sperm. Interestingly, distorters are not considered to act for the benefit of the individual carrying them and are considered “selfish genes.” Suppressors…well…suppress the distorters. The authors refer to these apparent Y chromosome suppressor/distorter regions as sex-ratio, or SR,  genes.

Dry season not a good sperm season

They also noted that during the dry season, buffalo didn’t make as much sperm, and the sperm they did make weren’t as frequently normal looking or very good swimmers. They hypothesize that semen quality may interact with the decreased availability of food in the dry season, leading to drop in Y haplotypes associated with a male-biased sex ratio. The investigative team, whose lead author, Pim van Hooft, is based at Wageningen University in The Netherlands, also suggested that the SR genes may be present in other species, adding a new dimension to the increasingly complex mechanisms of sex ratios in mammals.

For your consideration

1. Sex determination in vertebrates happens in a number of different ways. Some mechanisms don’t involve sex chromosomes at all but instead rely on environmental cues. Find an example of a species that uses environmental cues to determine sex. How can an environmental trigger be similar to a chromosomal trigger as a sex determinant? How do they differ?

2. Many species have life history strategies that involve adjusting sex ratios. What are possible explanations can you find to explain how adjusting sex ratio might benefit a species? How might it be a potentially dangerous gamble?

3. Distorters in general appear to be doing their host individual no favors. Given that fact, what is one explanation for the existence and persistence of suppressors of distorters?

Leeches model reproductive behavior

No, not that kind of modeling.

Leeches have a bad reputation because they dine on blood. Even forgetting for the moment such human-designed culinary delicacies as blood pudding or blood sausage, let’s just say that sucking blood does not necessarily an incubus make.

Not just blood-sucking boneless terrors

In fact, leeches have recently made a comeback in the shape–the slimy, creepy shape–of their use as medical therapy. Their former role was to suck bad humors from the body. Today, with our improved understanding of molecular biology and relegation of humor to Jon Stewart, leeches serve a different purpose. Pracititioners encountering venous insufficiency and premature clotting during certain surgeries can apply leeches–and their salivary anti-clotting factors–locally to address the problem. By the way, the medicinal use of leeches–which has a history stretching back for milliennia–is called hirudotherapy.

Model leeches

And leeches also make an oxytocin-related hormone called hirudotocin that plays a role in their reproductive behavior. A reproductively aroused leech, it seems, undergoes a maneuver that involves a sloooow, five-minute rotation of its body. The rotation results in alignment of reproductive pores with complementary pores on a presumably adjacent partner.

Animal behavior results, at its core, from an interaction of hormones and the nervous system. But linking the two directly and assessing the influence of hormones on nerves has proved elusive in more complex animals. Leeches, though, have a nervous system more basic than a mosquito’s. And an injection of hirudotocin yields leech reproductive rotation within minutes, accompanied by a leechy mouthing of the potential reproductive partner. In the world of animal behavior research, this is exciting stuff.

Sliced leech anyone?

To track the effects of this hormone through the animal’s nervous system, researchers at Caltech and UCSD examined nervous response to hirudotocin in slices of leech. Then, they did the ultimate direct assessment, removing all of the leech except the nervous system. This approach allowed them to trace directly the activation of the nervous sytem that led to the corkscrewing muscle movements of leech reproductive behavior.

Their next step will be to use voltage-sensitive dyes to detect electrical nerve signals along these paths to see which ones are involved in maintaining the behavior. They may not be drawing out bad humors any more, but leeches are certainly doing their part in helping us tease out the links between hormones and behavior.

For your consideration

Why is it so difficult to link a hormone and a behavior, especially in vertebrates?

This article says that animal behavior is a manifestation of the interaction of hormones and the nervous system. Can you think of some other examples of this interaction?

Animals are not the only organisms that use hormones. Plants do, too, but they lack a nervous system. Identify some plant hormones and determine what plant systems they influence.

%d bloggers like this: