Tag Archives: genetic variation

Uppsala post #1: Long-johns and Quantitative Genetics

Me in Uppsala (artist's impression).

Yesterday I arrived in Uppsala, Sweden, after a reasonably harrowing day which included getting up at 3.45am, and attempting to make small talk with a taxi driver on the way to the airport. Not only that, but I failed to read the email from the hostel which informed me of the minimal reception hours. This meant that I spent the first 2 hours of my time in Uppsala dragging my large suitcase around the city, through the snow, looking like a dick. After I got in and dropped it off, however, I went for a nice walk to the castle, gazed up at the stars, and slid down some snow-covered hills in a fever of excitement.

I may be 30, but that doesn’t mean that I’m not still AWESOME.

My reason for being here is not just so that I can have my opinion of long-johns changed forever (holy crap, they’re THE BEST), but also because I am taking a two-week quantitative genetics course being run by Dr Bruce Walsh of the University of Arizona (Note: replying to a taxi driver’s query as to whether you’re going to Sweden on holiday with, “No, I’m going on a two-week quantitative genetics course” is a guaranteed small-talk death blow).

I had planned to write short blog posts most days to cover briefly what he has been teaching us, but today has shown me the downright stupidity of such thinking. In just the first day, we’ve already covered pretty much everything I’ve struggled to teach myself over the past year and a half; one of the post-docs I chatted to at the morning break also informed me that he went on a 3-day course last year, and Walsh covered the entirety of that content in the first half hour.

So, yeah, it’s pretty intense.

Instead, I’ll give an extremely brief overview of what quantitative genetics is (and links to more comprehensive information), and hopefully continue along some of the basics – and how I am planning to use such techniques in my own research – over the next couple of weeks…

Quantitative genetics uses the insight that the expression of a single trait may be influenced by multiple genes, in addition to environmental factors, as a way to analyse phenotypic variation and evolution. Put simply, it enables us to study both nature and nurture! ‘Variation’ or ‘variance’ constitute almost every second word in the literature; we are never given the opportunity to forget that it is populations which evolve, not individuals, and therefore we are always interested in the phenotypic and genetic variation within populations.

There are a number of people who can be thanked for the development of this field, including Gregor Mendel, Francis Galton, Karl Pearson, and Sewall Wright, but one of the foremost is the statistician, biologist and all-round badass polymath that is Sir Ronald Fisher.

Word.

Anyone who has idly flicked through a biostatistics textbook (come on, we’ve all been there) will have seen mention of ANOVA; this ‘analysis of variance’ is based upon the concept of variance partitioning outlined by Fisher in his 1918 paper ‘The Correlation Between Relatives on the Supposition of Mendelian Inheritance’.

The very title of this paper gives you a good idea of what much of quantitative genetics entails: applying Mendelian principles of genetic inheritance in order to compare the phenotypes of individuals whose relatedness is known. The extent to which relatives resemble one another depends on how much the expression of the phenotype is determined by shared genes, as opposed to random environmental effects. The expression of a single phenotype (or a particular phenotypic trait) can be written mathematically as:

P = G + E

Where P is the phenotypic value, G is the genotypic value, and E is the deviance from this genotypic value caused by environmental effects.

Sounds pretty easy, right?

[EDIT: This video is supposed to start at the bit where Arnie says ‘WRONG’ and then shoots a guy in the face. It doesn’t, though. You don’t have to watch it. Oh, and if you haven’t seen Commando, *spoiler alert*]

Firstly, recall that we must deal with a population, so we have to think in terms of variation:

V(P) = V(G) + V(E)

The above are variances of phenotype, genotype, and environmental effects.

Next, we find that there are several components of genetic variation:

V(G) = V(A) + V(D) + V(I)

These components are additive genetic variance, dominance variance, and epistatic variance; only additive genetic variance is heritable, so this is the really crucial part. For more information on the others, and variance in general, I recommend this short paper in Nature.

Next up: environmental variation. These come in two broad forms – general environmental effects and special environmental effects – along with a special added bonus:

V(E) = V(Eg) + V(Es) + V(GxE)

But that will have to wait until another day, because it’s late and I’m tired and I’m going to bed.

Yes, probably still wearing my long-johns.

Of genetic variation and peacock spiders: Maratus volans and the lek paradox

Maratus volans, photographed by Jurgen Otto

This month saw the long-awaited publication in PLoS ONE of a paper describing the courtship of the peacock spider Maratus volans, a tiny arthropod whose displays have helped it achieve the heady heights of internet fame over the past couple of years (well, at least in those parts of the internet where people like to watch videos of little animals dancing around). Girard and her fellow researchers used high-speed video recordings and laser vibrometry to show that male spiders use vibratory signals in addition to ornamentation and motion displays in order to attract a mate.

I have written previously on how males and females invest different amounts of resources in their gametes (sperm and eggs), and how this imbalance creates the conditions for sexual selection – Darwin’s proposed mechanism for the evolution of different body shapes and sizes across the sexes. Sexual selection covers both female choice and male competition, scenarios that have led to the development of exaggerated male ornaments and weaponry respectively (consider, for example, the beautiful train of the peacock, or the fierce antlers of stags).

While weaponry is used to fight or simply intimidate opponents (as well as the rather ungentlemanly acts of prising rival males from females mid-copulation, and trapping females in mating burrows, as is the wont of some beetles), ornaments serve to impress and seduce the watching female. Highly-ornamented species are often those in which both sexes mate with multiple partners, with males offering nothing more than their sperm – not for them the worries of caring for offspring, or providing food and territory for their mate*. The displays that males engage in often serve to highlight their ornaments – male greater sage grouse Centrocercus urophasianus are a prime example:

This type of behaviour is especially evident in ‘lekking’ species, where males gather on a display ground (the lek) and parade their wares to potential partners. Only those males with the very best ornaments are deemed good enough by the choosy females, and each will likely mate with multiple partners – meaning that the genes of a select few sires are making it into the next generation. This leads us to the essence of the ‘lek paradox’: if females are selecting males on the basis of certain trait values, this should erode genetic variation in these traits, meaning that all traits should converge to similar values. If all traits were the same, females would be unable to choose between males, and – more importantly – there would be no point in trying to do so. I like to remember this paradox through the reappropriation of the lyrics to a popular song:



However, there is plenty of evidence to show that female choice on the basis of sexual traits persists. So, how can we explain the maintenance of genetic variation for sexual traits? One proposed mechanism is that ornaments develop a strong relationship with an individual’s ability to acquire resources from its environment and convert them internally to usable forms – a relationship known as ‘condition-dependence’. This ability includes factors such as fighting disease, catching prey, foraging, and metabolising nutrients. All the genes underlying these factors are associated with the sexual trait due to condition-dependence, and so the trait serves as an indicator of how the vast majority of an individual’s genome is performing in its current environment. Rather than eroding the variation in a few genes that encode a trait, selection is now based on the vast variation of virtually the entire genome. Not only that, but changes in the environment will alter which genotypes perform best, and mutations in any area of the genome will have some effect on mating success.

While the paradox is named after lekking species, which often provide the most extreme examples of ornamentation, the problem extends to all those species where males do not give their partners direct (or ‘material’) benefits. Research in this field helps us to figure out the wider effects of sexual selection – for example, can it help to prevent the build-up of deleterious mutations in a population? On a different level, it is interesting to ask why such behaviour exists – is sex really worth the male making himself quite so obvious to predators? How does a female ‘know’ which male is ‘good enough’? This paper gives us a nice description of the courtship behaviour that we see in this video, and provides a basis for further study of these charismatic little animals (and others in the genus Maratus) – this is especially intriguing as the ‘multi-modal’ nature of their courtship is ripe for further investigation. Each facet is a drain on resources, whether it be the development of the colourful abdominal flaps and ornamented third legs, or the waving and dancing itself – to say nothing of the vibrational drumming, wonderfully described as ‘rumble-rumps’ by the paper’s authors. Why have the males evolved these multiple signals? Do they represent different features of his quality, and can females discriminate between them? Is one signal more important than all the rest? I’m sure I’m not the only one who’s excited about what else this colourful spider can inform us about evolution.

Get the paper here.

See more videos from the Elias lab at Berkeley here.

Check out Jurgen Otto’s fantastic photographs here.

*Note: I could not find much detail in terms of the mating system in Salticidae, much less this particular species, so it may indeed be that males are providing females with direct benefits. In which case, ignore me.