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Searching for Speciation Answers: A Genetic Analysis

A species is defined as a group of organisms where two individuals can reproduce and have viable offspring (1). Stemming from this definition, researchers have held the belief that reproductive incapability between species occurred due to different environments, such as through geographic isolation (2). Rather than holding this theory as a fact, new research sheds light into understanding this issue better.

The University of Rochester and University of Nebraska have found a new driver for the inability of viable offspring between species, which was determined to be in the genetic makeup. Thier research gave new insight into what are called “selfish” genes or meiotic drive elements, which is the underlying reason if species converge or diverge as they evolve. It was determined that the sex chromosomes evolve faster than the rest of the chromosomes, which is what determines genetic incompatibility for reproduction. Chromosomes are divided into two types which consist of the sex chromosomes and autosomes. The sex chromosomes pass on an organism’s sex where the autosomes consist of the rest of the genetic information needed for the organism.

Mating between a species results in gene flow, which is the passage of genetic information and is an important factor for genetic variation (3). This gene flow is interrupted when two members of different species mate as the genes are unable to recognize each other due to the different species. The result of this is that the genes may not work properly as they should because they are interrupted by the presence of genes that are not able to work, which is vital for many functions such as reproduction.

Taking this concept, the researchers decided to build upon it through experiments where they crossed two different species of fruit flies. This cross resulted in female hybrid offspring (XX) that were fertile, but the males (XY) were sterile, which was the expected result as the XY sex becomes sterile first which ultimately leads to complete reproductive isolation. Mapping of the factors to explain the sterility, the researchers found the X chromosome to have more  incompatibility factors compared to the autosomes (2). Why did the X chromosome have this effect as compared to the rest of the genome? It ties back to the “selfish” genes.

“Selfish genes” are named due to their parasitic behavior where they propagate themselves at the expense of other genes, similar to a parasite. Meiotic drive elements therefore disrupt Mendelian inheritance which states half the offspring receive a gene from the parent. Meiotic drive elements change this allowing themselves to be passed on to more than half off the offspring, making it more prominent in subsequent generations. One of the researchers, Colin Meikeljohn said that “This could be because multiple meiotic drive elements from both parental species are unsuppressed in hybrids, and their combined action causes sterility” (2).

Rather than thinking meiotic drive elements ruin interspecies reproduction, it was found to also be responsible for promoting different species being able to reproduce. This is dependent on if gene flow is possible. In early speciation, reproductive incompatibility may not be found throughout the genome as some parts may still be compatible. During gene flow, a selfish gene can then be passed to the other population and spread by being passed down. The effect is that the genome will become exchangeable allowing viable offspring from the two species, according to Presgraves who was one of the researchers on the study (2).

Summing this together, meiotic drive elements are dependent on gene flow capability. If there is gene flow, it can cause convergence of species and allow for reproduction. If there is no gene flow, this causes divergence of the species as there is reproductive isolation (4). This research highlights the underlying role that genetics play in obtaining species and provides more insight into what notions were previously held.

Sources:

  1. “Species.” Merriam-Webster, Merriam-Webster, http://www.merriam-webster.com/dictionary/species.
  2. “What Makes Two Species Different?” ScienceDaily, ScienceDaily, 3 Jan. 2019, http://www.sciencedaily.com/releases/2019/01/190103120852.htm.
  3. “Gene Flow.” Merriam-Webster, Merriam-Webster, http://www.merriam-webster.com/dictionary/gene%20flow.
  4. Meiklejohn, Colin D, et al. “Gene Flow Mediates the Role of Sex Chromosome Meiotic Drive during Complex Speciation.” ELife, ELife Sciences Publications Limited, 13 Dec. 2018, elifesciences.org/articles/35468.

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