Inversion Immersion: How Chromosomal Architecture Leads to Speciation

What makes a species a species?  What are the characteristics that make it unique enough to be distinguished from similar creatures?  What are the genetic underpinnings that allow species to evolve to create a distinct entity?
 
These are the type of questions Dr. Carlos Machado is trying to answer. Answering them are key to understanding our planet's biodiversity. A traditional definition of speciation centers around the concept of reproductive isolation. While members of two different species may mate, if they either produce no offspring or sterile offspring, these two individuals represent different species.  This is known as the biological species concept. However, what factors lead to reproductive isolation? One way is through physical separation of populations (e.g., a mountain range) or allopatric speciation. On the other hand, what about sympatric speciation, or speciation in overlapping geographical distributions?

Fig 1: The famous Drosophila fruit fly.

These inversions have a lower recombination frequency which basically means the genes found in these regions do not change that much. Inversions can be found in many species including humans. Dr. Machado showed that species differences, such as mating discrimination by females and courtship song differences, mapped (are localized) to these regions. Fixed chromosomal rearrangements were critical for sympatric speciation of these two species. When compared to Drosophila species that do not co-occur in the same area (allopatric species), such as D. simulans and D. mauritiana, differences mapped to many different regions.

Building upon the shoulders of early giants in genetics, such as Richard C. Lewontin, John Lee Hubby, and Theodosius Dobzhansky, Dr. Machado’s work utilizes two species of fruit flies with geographical overlap (or sympatric distribution) in their populations: Drosophila persimilis and Drosophila pseudoobscura. He examined the chromosomal differences in each species to identify what makes each species unique. In these chromosomes, there are regions called inversions (Figure 2) where a segment of a chromosome is broken off, reversed, and then rejoined.

Fig 2: An example of a chromosomal inversion. Notice the segment in the middle is broken off, reversed, and rejoined. Image taken from: http://evolution.berkeley.edu/evolibrary/images/history/inversion.gif

Dr. Machado’s work brings us a step closer to understanding the roots of speciation. Following from this work, his future research ideas center on correcting inversions using the latest genetic techniques, such as CRISPR (Clustered regularly interspaced short palindromic repeats), to study in further detail these regions that are responsible for species’ differences.
 
Jonathan Wang is a PhD student in Raymond St. Leger’s lab studying host-pathogen interactions.
 
Jen Jones is a PhD student in Bill Lamp’s lab, studying how socioeconomic factors influence the distribution of mosquito populations.