Convergent Evolution of Silks in Arthropods

This article details a study regarding the convergent evolution of silk production in a few different arthropods. In particular, the study selected a type of butterfly, three types of caddisfly, and a type of spider. They each had different uses for their types of silk. They found that the variation in silk gene alleles are relatively consistent when compared between organisms which have long since been independently evolving. They suggested that this means there are common mechanisms for the formation of those genes within organisms. The researchers stress that they would like to repeat this study with other organisms which share similar traits but evolved independently. 

    The topic of convergent evolution is one that has always intrigued me. I am particularly interested in the pathways that evolution takes to develop similar traits despite independent evolution of organisms… As well as the fact that the above study could be replicated and utilized to study other traits relative to convergent evolution. Not only am I interested in the replication of this study, but the evolution of silk producing glands and maintenance of them in arthropods is such a cool concept. Overall, I look forward to seeing what other types of evolutionary genetic comparisons these researchers go on to make.

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. 

Horseshoe crabs are really relatives of spiders, scorpions

In an article from Science Daily, scientists used genetic data analysis and concluded that horseshoe crabs belong to the arachnid family tree. Arthropods are considered to be the most successful animals on the planet and the group include insects, crustaceans, and arachnids. Although horseshoe crabs are marine animals and known to be in the arthropod family, their genomic data have repeatedly shown their relationship with arachnids like spiders, scorpions, ticks, and mites. Horseshoe crabs are the only marine animals to breathe with book gills which is similar to the book lungs spiders and scorpions in land use to survive. There are only 4 species of horseshoe crabs that are alive today. According to the fossil record, the first group appeared 450 million years ago with extinct lineages like scorpions. In order to test the complex relationship among horseshoe crab with other species, scientists from University of Wisconsin-Madison compared the genomic sequence of the three out of four of the alive horseshoe crab against the genome sequence of 50 other arthropod species. As a result of the analysis, the research revealed that horseshoe crabs nested in the arachnid family.

Some scientist might not agree with the findings of this study because of the difference in colonization of the two species of animal. Horseshoe crabs are water dwellers while the arachnids colonized land. These animals belong to a group called Chelicerata and possibly have a common ancestor. I do think that more research with major characteristics of the two species needs to be done with major in order to support the data of this study.