Marronoids: The (Mostly) Little Brown Spiders With a Big Story to Tell

By Jacob A. Gorneau, Katherine O. Montana, and Lauren A. Esposito, Ph.D.

Museum collections are libraries of the world’s biodiversity that hold stories about evolution, global change, and even disease or invasive species. Until recently, these stories have usually been told in the context of morphology, or the anatomical structures that organisms possess. However, with DNA sequencing technologies now widely available, we can extract and sequence even the damaged DNA of older specimens held in these collections, unlocking a whole new section of this proverbial library. This ability is especially helpful in groups that are “cryptic,” with species difficult to identify using morphology alone.

It may be the birdwatching community that coined the phrase “little brown jobs” to describe hard-to-identify organisms (similar to “damn yellow composites” in plants), but little brown jobs—or LBJs, for short—is now commonly applied to bats, moths, fungi, and even spiders! The phrase is often used pejoratively, though those who study LBJs may use the phrase with a glint of admiration. Likely due in part to their nondescript features, LBJs are often understudied when compared to their larger, more colorful counterparts.

One group of LBJs are the “marronoid” spiders. These spiders were not previously thought to form a single monophyletic group, or a group with shared evolutionary history culminating in a common ancestor. Morphology-based classification schemes placed the families that belong to this group in different places on the tree of life, and it wasn’t until the first spider tree of life was built using genetic information in 2017 that these seemingly unrelated spider families were united in a single branching lineage. This was surprising because the collective group had no identifiable morphological synapomorphy—or unique shared characteristic that can be used to identify a group with a common ancestry.

The authors of that publication named the group the “marronoid clade” after the Spanish word marrón, meaning brown—though not all species in the group are brown! It seems that historically, many small, brown, nondescript spiders have been assigned almost haphazardly to the group, and although there is an understanding that these spiders are related to each other, the nature of these relationships is still unclear.

Why study LBJs? Color and size are only two axes of diversity. The marronoid clade is host to about 3,400 spider species—half the number of all known mammals! Furthermore, these spiders exhibit a range of diverse ecologies and behaviors that are not often seen in the spider tree of life. The group includes social species, intertidal species, species that can tolerate the harsh cold of Siberia, others that can tolerate the oppressive heat of Death Valley, and the only known fully aquatic spider, Argyroneta aquatica. Yes, these spiders may be (generally) brown and (generally) small, but they are incredibly interesting, and we have a lot to learn about spiders, and evolution more broadly, from them.

As part of a research team at the California Academy of Sciences, supported by a National Science Foundation grant awarded to Sarah Crews, Ph.D., and Lauren Esposito, Ph.D., we sought to understand the relationships of families in the marronoid clade using genomic data—or all of the genetic information (DNA) contained in the body of an organism. Previous to genomic sequencing, researchers sequenced individual genes in a process called Sanger sequencing. While cost-effective in the short run, it provides less data and is more expensive overall. Furthermore, we knew from the previous spider tree of life study that the genes used in Sanger sequencing didn’t provide enough data for us to parse the relationships we wanted to investigate.

We had to overcome a few challenges when it came to sequencing the genomes of these spiders. Most of this work was conducted in the immediate wake of the COVID-19 pandemic, preventing international travel to collect fresh spider specimens. Consequently, we had to rely on specimens from museum collections that tend to have more damaged or degraded DNA as a result of break-down over time. Since many spiders in this group are very small, just a few millimeters at best, we had the added problem of a very limited amount of tissue that we could extract DNA from.

Our solution was to use low-coverage whole genome sequencing (lcWGS) on museum collection specimens, which allows us to sequence indiscriminately most of the DNA extracted from these spiders. When we received our sequence data, we then computationally “harvested” ultraconserved elements (UCEs) from the data, which are parts of the genome that are especially conserved in places but are variable in others. This mix is helpful in phylogenetics (tree building) to help understand the “backbone” of the tree or relationships deeper in time (for example, between families), as well as the relationships between species more closely related. This “harvesting” step essentially is a search command that allows us to find parts of the genome that are the exact same section in each specimen.

In addition, we also searched our genomes for the individual Sanger genes that had been used in previous studies. By doing so, we could incorporate published data available freely online and increase the number of species we could include in our study. These data, as well as the data generated for our study, are available for anyone anywhere in the world to download and incorporate in their own research through public genomic repository sites like GenBank and the Sequence Read Archive.

Through this study, we inferred the most complete phylogeny (tree) of the marronoid spiders with high statistical support for relationships, from nearly 1,300 UCEs and six Sanger genes. Based on the results, we realized that some genera needed to be moved into different families, that a group of mostly cave-dwelling spiders (Cicurinidae) needed to be re-elevated to family, and that a well-known subfamily of spiders needed to be recognized as a distinct family (Macrobunidae) for the first time.

In terms of future directions, stay tuned for additional research from the Arachnology Lab at the California Academy of Sciences—we are wrapping up a study on one of the largest, and possibly messiest, marronoid families, Dictynidae. This family has its fair share of LBJs, with plenty of lifestyle diversity within it, and there is a lot of taxonomic work to be done to begin to fix this LBJ mess. Like the rest of the marronoids, uncovering the evolutionary relationships of dictynids has been based heavily on museum collection specimens and low-coverage whole genomes, proving that the utility of these valuable collections is more than just morphology. We see a future in which methods like ours can unlock the drivers behind spider and insect biodiversity broadly!

Jacob A. Gorneau is a Ph.D. student in the Department of Entomology at the California Academy of Sciences and in the Department of Environmental Science and Policy Management at the University of California, Berkeley. Email: jgorneau@calacademy.org. Katherine O. Montana is a research assistant in the Department of Entomology at the California Academy of Sciences and a graduate of the integrative biology master’s program at San Francisco State University. Email: kmontana@calacademy.org. Lauren A. Esposito, Ph.D., is an Assistant Curator and Schlinger Chair of Arachnology at the California Academy of Sciences. Email: lesposito@calacademy.org.