Our chromosomes are like socks: you want to have a pair of them, nothing more and nothing less. We have 23 pairs of chromosomes per cell, and there are great costs to exceeding this ideal number. Having odd numbers of certain chromosomes leads to genetic disorders like Down syndrome, while babies with three of every chromosome – triploids – tend to be lost to miscarriage or die within months. But having extra chromosomes isn’t always bad. In our liver, it’s positively encouraged.
Cells with extra chromosomes are known as polyploids and they’re a common feature in all mammalian livers. Some have four copies of each chromosome; others have eight, even sixteen. Now, Andrew Duncan from the Oregon Health and Science University has found that liver cells can cycle through chromosome numbers with surprising ease, frequently increasing and reducing their counts.
In Robert Louis Stevenson’s classic story, Dr Henry Jekyll drinks a mysterious potion that transforms him from an upstanding citizen into the violent, murderous Edward Hyde. We might think that such an easy transformation would be confined to the pages of fiction, but a similar fate regularly befalls a common fungus called Fusarium oxysporum.
A team of scientists led by Li-Jun Ma and Charlotte van der Does have found that the fungus can swap four entire chromosomes form one individual to another. This package is the genetic equivalent of Stevenson’s potion. It has everything a humble, Jekyll-like fungus needs to transform from a version that coexists harmlessly with plants into a Hyde-like agent of disease. In this guise, it infects so many plant species so virulently that it has earned the nickname of Agent Green and has been considered for use as a biological weapon. It can even infect humans.
These disease-making chromosomes came to light after Ma and van der Does sequenced the genome of a variety of F.oxysporum called lycopersici (or ‘Fol’), which infects tomatoes. Its genome was unexpectedly massive, 44% bigger than its closest relative, the cereal-infecting F.verticillioides. Looking closer, Ma and van der Does found that most of this excess DNA lies within four extra chromosomes, which Fol has and its relative lacks. Together, they make up a quarter of Fol’s genome.
Ma and van der Does demonstrated the power of this extraneous quartet by incubating a harmless strain of Fol with one that causes tomato wilt. Just by sharing the same space, the inoffensive strain managed to acquire two of the extra chromosomes found in the virulent one. And, suddenly, it too could infect tomatoes. In a single event, the fungus had been loaded with a mobile armoury and changed into a killer. It seems that the fungus needs just two of the four chromosomes to cause disease; the others probably act as accessories, boosting its new pestilent powers.
The animal on the right is no ordinary chicken. Its right half looks like a hen but its left half (with a larger wattle, bigger breast, whiter colour and leg spur) is that of a cockerel. The bird is a ‘gynandromorph‘, a rare sexual chimera. Thanks to three of these oddities, Debiao Zhao and Derek McBride from the University of Edinburgh have discovered a truly amazing secret about these most familiar of birds – every single cell in a chicken’s body is either male or female. Each one has its own sexual identity. It seems that becoming male or female is a very different process for birds than it is for mammals.
In mammals, it’s a question of testicles, ovaries and the hormones they produce. Embryos live in sexual limbo until the sex organs (gonads) start to develop. This all depends on a sexual dictator called SRY, a gene found on the Y chromosome. If it’s present, the indifferent gonads go down a male route; if not, they take a female one. The sex organs then secrete a flush of hormones that trigger changes in the rest of the body. The sex chromosomes are only relevant in the cells of the gonads.
But the gynandomorphs show that something very different happens in birds. Birds have Z and W chromosomes; males are ZZ and females are ZW. Zhao and McBride used glow-in-the-dark molecules that stick to the two chromosomes to show that the gynandromorphs do indeed have a mix of ZZ and ZW cells. However, they aren’t split neatly down the middle. Their entire bodies are suffused with a mix of both types, although the male half has more ZZ cells and the female half has more ZW ones.
Even though the three chickens were both male and female, one of them only had a testicle on one side, the second only had an ovary on one side, and the third had a strange hybrid organ that was part testis and part ovary. These malformed organs pumped the same soup of hormones throughout the birds’ bodies but, clearly, each side responded differently.
Zhao and McBride started to suspect that each cell has its very own sexual identity, and that this individuality exists from the chicken’s first days of embryonic life. They proved that by transplanting cells from embryonic sex organs from one animal to another. All the transplants produced a glowing green protein so Zhao and McBride could track their whereabouts, and those of their daughters.
Whiptail lizards are a fairly ordinary-looking bunch, but some species are among the strangest animals around. You might not be able to work out why at first glance, but looking at their genes soon reveals their secret – they’re all female, every single one. A third of whiptails have done away with males completely, a trick that only a small minority of animals have accomplished without going extinct.
Some readers might rejoice at the prospect of a world without males but in general, this isn’t good news for a species. Sex has tremendous benefits. Every fling shuffles the genes of the two partners and deals them out to the next generation in new combinations. Sex creates genetic diversity and in doing so, it arms a population with new weapons against parasites and predators. These benefits are so big that sex is nigh universal among complex life. Only a few groups, like the incredible bdelloid rotifers, have found ways of becoming permanently asexual.
Doing away with sex is even rarer for vertebrates (back-boned animals). The whiptails of the genus Aspidocelis are a flagrant exception. Their forays into asexuality started when two closely related species mated. For some reason, these encounters produced asexual hybrids. For example, the New Mexico whiptail (Aspidocelis neomexicana) is a hybrid of the Western whiptail (A. Inornatus) and the little striped whiptail (A. tigris). In the hybrid species, the females (and there are only females) reproduce by laying eggs that have never encountered any sperm.
The problem is that this really shouldn’t work. Sperm and egg cells are created through a process called meiosis, where a cell’s chromosomes are duplicated before the cell divides twice. This produces four daughter cells, each with half the DNA of the original. This means that egg cells only contain half the total number of chromosomes that most other cells in the body do. It’s their union with sperm, which are also genetically half-cocked, that restores the full balance of chromosomes, ready for the next generation.
So how do the lizards get their full set? The answer is deceptively simple. They start off with twice as many.
For humans, sex is a simple matter of chromosomes: two Xs and we become female; one X and a Y and we develop into males. But things aren’t so straightforward for many lizards – many studies have found that the temperature of the nest also has a say, even overriding the influence of the chromosomes. But the full story of how the lizard got its sex is even more complicated. For at least one species, the size of its egg also plays a role, with larger eggs producing females, and smaller ones yielding males.
The discovery comes from Richard Shine’s group at the University of Sydney. In earlier work, they showed that if the Eastern three-lined skink (Bassiana duperreyi) incubates its eggs at low nest temperatures, XX carriers develop into males regardless of their chromosomes.
Now, Rajkumar Radder, a former member of Shine’s team, has shown that the amount of yolk also determines the sex of a skink, but only at low temperatures. By deliberately adding and removing yolk from eggs using a syringe, he managed to alter the sex of the hatchlings. This degree of complexity is totally unprecedented – it means that for the skink, sex is a question of its chromosomes, the temperature it was reared under and the amount of yolk it had.