Sunday, March 20, 2011

Searching for the genetic basis of adaptation in Bees

Genetics of Adaptation Graduate Seminar
Author: Priya Date

It is a common view among scientists who study human evolution that humans originated on the African continent and then migrated. Isn’t it surprising how different people from different parts of the world look? Not only do we look different, but also we can have very different styles of living. One important question is, if these differences are culturally transmitted, or genetically heritable, i.e., we pass them down to our kids through our genes (DNA). However, because of our long life spans, late age of reproduction and difficulty to conduct genetic experiments in humans, it is very difficult to study these kinds of questions. This is when other organisms like the little buzzing critter ‘the honey bee’ can come to our rescue.

The honeybees also originated in Africa and have then expanded in range to Asia and Europe due to introduction by humans. These bees are thus now living in different parts of the planet (thanks to us !) and show region-specific adaptations in how they look or behave. Bees in Europe that had genes that helped them to survive in the cold climate reproduced and thus passed on the heritable component of their DNA for cold survival to the next generation. The same is true for why the bees from Europe look different from the African bees. Even though we have some indication that some genes are involved in adapting to new environments, we still don’t know what regions of the whole genome of any organism are important for these adaptations. In this study by Zayed and Whitfield (PNAS article, 2008) scientists have taken the opportunity to take advantage of the sequenced genome of honeybees to look for signature of selection. By signature of selection we mean whether changes in certain parts of the genome are increasing in frequency in the population in a geographic location specific manner. This would then indicate that those genomic regions have some adaptive value to increase the bee’s fitness or survival in any given environment.

Before we delve into the details of this study lets take a few moments to refresh our basics of molecular biology. The genome of any organism is a huge string of DNA. The DNA sequence is made up of four bases which are read as A, T, G and C. As we make words by putting together certain alphabets in the right sequence, the DNA sequence when put in the right order makes genes, which encode proteins. And just like changing the sequence of alphabets in a word will make it meaningless, changing the sequence of DNA makes different proteins that might be non-sense proteins. Very rarely, these changed or damaged proteins in turn may result in changing how we look or behave. We only store the changes that are beneficial and generally get rid of those which are harmful. However, genes form only a small proportion of the whole genome, sometimes as little as 1% in certain organisms: much of it may code for nothing but is important for regulating the expression of the coding region. However, in this study the authors focus on only the coding regions. Thus if we want to find out if certain regions of the genome have changed to suit the environmental needs we can compare the ancestral genome with the derived or newer genome and screen for signatures of selection.

Zayed and Whitfield have compared certain genomic regions of the African, Asian, East and West European bees for changes in the DNA sequence. To do such comparison researchers have developed very complicated statistic called ‘Fst’. In simple words, it goes from 0 to 1 where 0 indicates no genetic differences between if two DNA sequences and 1 indicate a lot of differences in the DNA. Thus, if the derived genome were to be very similar to the ancestral genome its Fst would be 0 and if it were diverged the Fst would be 1. In the bee study, the authors found that the West European population of bees was significantly diverged from its ancestral African population, but the other two populations, however, do not show any evidence for divergence at this stage (Figure 1). This may be due to the fact that the Asian and East European environment is somewhat similar to the African environment. This data suggest that the vast differences seen in the West European and African bees has a strong genetic basis and more functional studies relating the two should be performed. 

Finally, turning back to the questions we started with to understand how the we look different, studies like this can provide some common rules or generic principles which will hold true anywhere. As the bees, which originated on the African continent when moved to the colder climate of West Europe changed in order to survive in the new environment, humans have also evolved several adaptations. For example, a study shows genome-wide signatures of selection acting on certain pigment related genes in Asian, European and African people (Genetics-of-human-pigmentation). This can be explained as the people inhabiting the tropical regions have darker skin pigment to gain protection from the uv radiation and thus have diverged in skin color from their European counterparts. Not only skin color but several such examples leading to difference in our height, stature, behavior, etc. can be found which probably have similar genetics and animal models like the honey bees can be used to understand these genetic mechanisms.

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