Researchers Found the Genes Responsible for Changing Ground Beetle Genitalia

Japan's Carabus male beetles mate with females by inserting their chitin-covered appendage. Two pieces of the appendage break off: a sperm delivery tube and a copulatory piece. Researchers identified several genes that control the size of the male beetle's "piece" and the female beetle's pocket. Initial hypotheses demonstrated that the male and female's genes for genitals should be connected through coevolution which should show in their genome. In other words, the size and shapes of genitalia in species should be analogous because the same genes influence male and female dimorphism. However, researchers found that there is much genetic diversity between the genes that determine these beetle's genitalia sizes. The discovery is a significant case in speciation because it contributes to an old speciation model called the "Lock-and-Key Hypothesis".

The Lock-and-Key Hypothesis is a method of reproductive isolation, specifically mechanical isolation which separates a species from mating. For example, the beetles described in the article have such a large genetic diversity contributing to genitalia that some of the male appendages do not fit into females. The article states, "Out-of-sync sizes can cause ruptures, snap-offs and generally low numbers of offspring." Talk about painful! These beetles are now contributing to hybridization which can eventually evolve into a whole other species altogether. Genetic hybrids tend to be infertile, but researchers found that the hybrids of two different beetle species in Japan can produce offspring.





It is truly interesting to see evolution in motion especially with the Lock-and-Key Hypothesis. It's no secret that genitalia in male species may not fit into a female in either the same species or another. The Lock-and-Key Hypothesis was proposed over a hundred years ago, but it's often overlooked because other reproductive isolations such as gametic isolation or chromosomal differences are better modes of preventing hybridization. The discovery here is fascinating because genetic diversity actually prevented some of these beetles from mating within their own species.

Sources:
https://www.sciencenews.org/article/ground-beetle-genitals-genetics-evolution-battle-sexes
https://advances.sciencemag.org/content/5/6/eaav9939

Genetics of Genitalia: he have one but lizards have two

Researchers at Harvard Medical School have been able to crucially understand why reptiles have genitalia structural different but functionally similar to that of mammals and birds. Snake and lizard genitalia are derived from tissue that gives rise to the legs while mammalian genitalia are derived form tissue that give rise to the tail bud. It turns out that the embryonic cloaca, which later becomes the urinary and digestive tract, issue molecular signals that tells neighboring cells to form the external genitalia. Since location determines which tissue receives the signal first it is different in mammal and reptiles. To confirm this the researchers grafted cloaca tissue next to the limb bud in one group of chicken embryos and next to the tail bud in another group. They found that in both cases cells closer to the grafted tissue partially converted towards a genitalia fate. Their findings confirmed their hypothesis that different populations of cells with progenitor potential are able to respond to cloaca signals and contribute to genitalia growth.

vestigial limb bud and hemipenis of an embryonic snake

The research was interesting and conclusive in showing that even though the genitalia is derived differently in mammals and reptiles, they are homologous in that they are derived from the same genetic programming.

Main article: http://www.sciencedaily.com/releases/2014/11/141105131941.htm

Related article: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13819.html

Lizards have two genitalia. While We Have one. Why?

Researchers in Harvard Medical School's Department of Genetics, led by departmental chair Clifford Tabin were on a quest to find out why and how snakes and lizards have two genitalia while birds and people have only one.

It is noted that snakes and lizards have their paired structures located somewhat at the level of the limbs, while ours, and the birds', are located a bit further down. Snakes and lizard genitalia are derived from the tissue that gives rise to hind legs, while mammalian genitalia are derived from the tail bud. But despite such contrasts, these structures are functionally analogous and express similar genes. Researchers found that it was it all due to the location of cloaca.

Python embryo at 11 days after oviposition

The embryonic cloaca issues molecular signals that would tell the neighboring cells and tissues to form into external genitalia. It is the cloaca’s location that determines which tissues get the signals first. In snakes and lizards the cloaca is located closer to the lateral plate mesoderm, the same tissue that makes the paired limbs, receives the signal. While at the same time in mammals it is closer to the tail bud.

To further confirm this theory the researchers grafted cloaca tissue next to the limb buds in one group of chicken embryos and besides the tail buds in the second group. It was found by the team of researchers that in both the cases the cells that were grafted closer to the cloaca responded to the signals and partially converted towards a genitalia fate. This proved that different cell populations that have potentials respond to the cloaca signals and contribute towards the genitalia outgrowth.  

Clifford Tabin said that mammals and reptilian genitalia are not homologous as they are derived from different tissues but at the same time they both share homology in the sense that they are derived from the same genetic program and induced by the same parental set of molecular signal. "Moreover, this might help to explain why limbs and genitalia use such similar gene regulatory programs during development," said Patrick Tschopp, an HMS research fellow in genetics in Tabin's lab and first author on the paper. He also added that how, “evolutionary shift in the source of a signal can result in a situation where functionally analogous structures are carved out of nonhomologous substrate."

This research throws a lot of light on the evolutionary aspects of the mammalian and reptilian genitalia. Also the fact that the cloaca initiates the genitalia outgrowth is very interesting and surprising to know. If a more in depth research would be done I think it could reveal some more genetic linkages between mammalian and reptilian traits. 

Article: http://www.sciencedaily.com/releases/2014/11/141105131941.htm

 Related Article: How do snakes mate - http://animals.howstuffworks.com/snakes/snake5.htm