Subtle deviations from Mendelian expectations point to a meiotic driver in the Timor subspecies.
Every biology student has worked his or her way through the pea-experiments of Gregor Mendel, creating Punnett squares with recessive and dominant alleles. One of the most important insights from these pivotal experiments was the observation that every allele at a genetic locus has an equal probability of being transmitted to the next generation (ultimately giving rise to the predictable ratios of dominant and recessive traits). However, some alleles show clear deviations from these expected patterns. These genetic elements are known as meiotic drivers, because they “drive” the meiotic cell division process in such as way that they have a higher chance of ending up in the gametes (i.e. eggs or sperm cells).
During meiosis, the chromosomes are sorted into four daughter cells. In birds, one daughter cell develops into the mature egg, while the other three develop into polar bodies. The spindle apparatus attaches to the centromeres of the chromosomes and drags them into the different daughter cells. Meiotic drivers are often found close to centromeres, because this allows them to influence the spindle fibers and ensure that they end up in the daughter cell that becomes the mature egg. Indeed, previous research in chickens reported a meiotic driver at the centromere of chromosome 1.
Subspecies
Meiotic drivers can be difficult to detect, because they tend to be transient phenomena. In some cases, the meiotic drivers lead to deleterious effects and are quickly suppressed by other genetic elements that restore proper Mendelian segregation. Alternatively, meiotic drivers are so successful that they rapidly spread through a population and become fixed (i.e. all individuals have the same genetic variant). One way to identify such cryptic meiotic drivers is to cross individuals from divergent populations. If a meiotic driver evolved in one population but not the other, it will become visible in the hybrids. A recent study in the journal Ecology and Evolution used this approach to look for meiotic drivers in the Zebra Finch (Taeniopygia guttata). The researchers crossed two subspecies – Australian (castanotis) and Timor (guttata) Zebra Finches – and traced the genetic ancestry of several molecular markers. Did some deviate from the expected Mendelian patterns?
Backcrosses
The experiments revealed “no clear evidence for any active meiotic driver in a cross between Australian and Timor Zebra Finches.” However, there was a significant deviation from Mendelian segregation in females of the first backcrossed generation. This finding suggests that there might be a weak meiotic driver which allows Timor centromeres to outcompete Australian variants in the race for the oocyte. It took numerous hybridization and backcrossing events to detect this subtle signal, indicating that weak meiotic drivers might be more common than we think. Another example of how avian hybrids can lead to exciting discoveries and new insights.
References
Knief, U., Forstmeier, W., Pei, Y., Wolf, J., & Kempenaers, B. (2020). A test for meiotic drive in hybrids between Australian and Timor zebra finches. Ecology and evolution, 10(23), 13464-13475.
Featured image: Zebra Finch (Taeniopygia guttata) © Peripitus | Wikimedia Commons
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