Horseshoe Crabs, Spider Cousins or Something Else?

Horseshoe Crabs represent a long and proud lineage of chelicerate arthropods. Famed for their role as survivors, the order Xiphosura (fancy word for horseshoe crabs) can trace their fossil record back to the Ordovician Period some 445 million years ago. A once diverse lineage of animals, today xiphosura is represented solely by the Atlantic Horseshoe Crab Limulus polyphemus. Traditionally it has been hard to place the Horseshoe Crab into the evolutionary tree of life. Many studies have concluded that the Horseshoe Crab is a distant relative of arachnids. However many of these studies were going in with a biased assumption, arachnids are a monophyletic group of organisms. 

An Atlantic Horseshoe Crab

In evolutionary biology, phylogenies are created using various datasets to reconstruct the evolutionary relationships between different organisms. When a selected group of organisms can trace their heritage back to a common ancestor, we call this group monophyletic. A good example of a monophyletic  group is the Primate Order. All living primates share a common ancestor as consistently indicated by both fossil/morphological datasets as well as genetic datasets and are therefore a monophyletic group of organisms. Things can get messy with recovering the phylogenies of organisms with broad and unclear relationships that originated deep in time. Such is the case with chelicerates.

Simplified Taxonomy of Tetrapods 

Note: You are indeed a fish, no disrespect

You and I are both fish, in a taxonomic sense, as are all terrestrial vertebrates. We can trace our ancestry back to boney fishes in the Devonian and many features we take for granted, such as our ears and Adams-apple are highly derived gill ridges. Obviously though, when I tell you that I'm going to spend a weekend fishing, you the reader aren't confused by evolutionary semantics and envision a set group of animals. Perhaps if I say I'm fishing at a lake, you the reader would envision me reeling in a trout or pike. If I say I'm going to fish at sea perhaps you the reader envision me reeling in a tuna or a shark. You don't however, envision me catching an elephant nor a kangaroo when I say "I'm going fishing" despite both organisms being *fish* in the evolutionary sense as their ancestry is ultimately bound to the water. The same applies for reptiles, most people generally view reptiles and birds as separate categories of organisms. This division between birds and reptiles falls apart when one considers that birds are dinosaurs and dinosaurs are clearly reptiles. Paraphyly refers to when  a large group of animals are given a name/rank but selected subgroups are omitted due to differences in bodyplan or ecological habit. So whenever you call something a "fish" but are failing to include every other terrestrial vertebrate in your referral, you are being what taxonomists call "paraphyletic". 

Simplified Taxonomy of Birds

Note: Crocodilians are the closest living relatives of birds

Chelicerates should be defined before we go any further as things could get confusing quickly. Today the chelicerates include the following; mites, spiders, sea spiders, harvestmen, scorpions, pseudoscorpions, camel spiders, whip scorpions.... You get the point. The exact relationships between the sub-categories of the chelicerates is a bit messy, it was long assumed that chelicerates include a monophyletic "arachnid" sub-group with xiphosura being its sister out-group clade. Genetics has brought into question that assumption. Most morphological studies recovered a monophyletic arachnid group with a sister monophyletic Xiphosura clade. 

Genetic analytical work by Ballesteros et al. has shown however that many of the "arachnid" groups are are not related closely to one another and are instead convergent. More interestingly the study found Xiphosura to be nested smack-dab between Opiliones (harvestmen/daddy longlegs) and Pseudoscorpions both of which are traditionally considered arachnids. This result is interesting as it implies that the traditional definition of "arachnids" is a paraphyly. To recover arachnids as a true monophyly, the definition would need to be expanded to include  Xiphosura, meaning that Horseshoe Crabs are arachnids. 

Phylogenetic tree from Ballesteros et al. 

Scientists genetically modify daddy long legs to have short legs

 

Researchers from the University of Wisconsin-Madison have sequenced the genome for the Phalangium opilio, also known as the daddy long leg spider. The ultimate goal of the study was to learn more about arachnid genome evolution and how these genes gave rise to such features as fangs and pinchers. While researching this, they took the genome and used RNA interference to turn off the genes associated with leg development in these spider embryos. The result was that six of their eight legs came out shorter than normal and were closer to a pedipalp appendage, which is used for food. The researchers next plan is to develop the first transgenic P. opilio, from this genome.

https://www.cnet.com/news/scientists-tweak-daddy-long-legs-genes-to-create-daddy-short-legs/

https://www.genome.gov/genetics-glossary/Genetic-Engineering

Spiders That Aren't Dying to Reproduce

It is commonly known that there is sexual cannibalism in mating, especially with specific species of spiders. Female spiders can decide to eat the males before, during or after mating. Male spiders have tired to find different ways to distract the female to avoid being eaten, which has led to a new type of defense. They tie up the female before sex with their spider silk so they can escape before the female gets free. Alissa G. Anderson, a graduate student in biology at the University of Nebraska, said that she had not witnessed any cannibalism in the nursery. This could have been because these spiders always wrapped up the females so there was never a chance for them to be eaten. Males with shorter legs and ones that couldn’t produce silk were eaten more often. Results have shown that females who are wrapped up have a higher number of eggs which has benefits in both sexual and evolution terms.    

Cobwebs Hold Genetic Secrets About Spiders and Their Prey

Scientists have discovered traces of DNA stuck on old spider webs. DNA from both the spider and any other creature that has fallen trap to the spider’s web can be collected from old spider webs. These traces of DNA can be of importance to researchers in the fields of conservation ecology, pest management, etc. Scientists amplified cytochrome oxidase 1, a mitochondrial gene, in order to identify a species that may have been on a spider web. This type of technology allows researchers to collect a number of spider webs throughout an environment to possibly detect the survival of endangers species. It also makes identifying species a whole lot easier and humane when you don’t have to capture and kill an organism.

            I personally believe this type of technology is great. I believe in the preservation of natural wildlife, and to be able to easily identify species of animals in a given environment just by a spider web makes it good for conservation ecologists to work. It is also, however, much more humane to organisms when they don’t have to be stripped from the wild and killed. This article was very interesting, and I would like to further investigate how this type of DNA research is benefiting our wildlife.

Original Link

Spider Silk Utilized for Gene Therapy

An article on Science Daily discusses a study conducted by David Kaplan and other scientists.  It explains that the idea of gene therapy has been studied for quite some time (over 1,500 trials since 1989).  Although the idea of gene therapy is not new safety is the main concern.  The idea of utilizing genetically altered viruses to treat patients has been studied; however, it has not been approved by the FDA.  Kaplan and other scientists proposed the idea to utilize silk proteins from spiders to transport genes.  The silk proteins were modified to attach to unhealthy cells and glow(same gene as fireflies have), in order to "pinpoint" where the gene is.  An experiment involving modified silk protein took place with mice who had human breast cancer cells.  According to Science Direct as well as News-Medical.net it appeared the spider silk injected its DNA into the cancerous cells and did not harm the mice at all.  News-Medical states, "genetically-engineered spider-silk proteins represent a versatile, very highly tailorable and useful new platform polymer for non viral gene delivery".  The idea of treating cancer with silk extracted from spiders is mind blowing.  The discoveries made by scientists in gene therapy is very impressive.  Perhaps with more clinical studies Kaplan and other scientists will be able to convince the FDA that there are safe and effective.