Phylogeography of two species in the "neglected" taxon of mygalomorph; the Funnel web spiders of Austrailia



Dinner plate for scale!

Spiders are massively cool creatures! Reasons for this include big pointed (sometimes poisonous) fangs (Chelicerae), or they can get to the size of dinner plates (including leg span). Mygalomorph (Infraorder) is an infamous taxa of spider which includes the likes of trapdoor spiders and Tarantulas. They also include the spider of interest in this paper the Funnel web spider! Most phylogeographic studies look at how varying environmental conditions or dispersal barriers cause speciation, in certain related taxa. The reason this paper caught my attention was due to their reference too ‘neglected taxa’. This was mainly due to the hypothesis they were testing. This was how two niche differentiated spiders in the same taxa that are sympatrically dispersed, can have such variation in evolution and population structure under the same environmental conditions. 

 

Unfortunately with phylogeographic research there is a reliance on large, multilocus datasets. These data sets are usually derived from past research on high profile taxa. Interestingly proposed by the authors of this paper, is that these species do not paint the whole proverbially picture. Taxa like terrestrial invertebrates have characteristics like longevity and persistence in the environment, which can retain and display phylogeographical signals over a longer time period than “obvious” groups (Probably stupid overrated groups like mammals!!). These obvious groups get sucked into the ‘snowball down a hill’ theory (research spawns more research) leading to a noticeable absence of research on possibly more interesting taxa. Initial pioneer studies are unattractive however due to initial ground work required! Collection of understudied taxa is difficult for obvious reasons (Where the heck do I find these things!). Molecular techniques can be highly specialised so for some taxa, development of the methods to extract genetic information can take a long time. With enough determination and perseverance these barriers can be overcome to show some really remarkable phylogeographic patterns.

 

So what’s the deal with these funnel web spiders? Well for one they are distinguished from another common group of spiders the Araneomorphae. Mygalomorphs only respire via booklungs compared to Araneomorphae which respire via booklungs and trachea. The booklungs expose a large surface area to the air making them highly susceptible to desiccation. This leads to a sedentary lifestyle, living in burrows and expressing a low vagility (movement in the environment!). These characteristics mean that the mygalomorphs can retain phylogeographic signal at finer scales over long time periods. Recent work has been completed revising the taxonomy of the Australian funnel web spiders. The two species of interest in this study are from the family Hexathelidae, and are called Hadronyche cerberea (Koch, 1873) and Atrax sutherlandi Gray, 2010. Both species have largely overlapping ranges and distributions in SE Austrailia, also being sympatrically and continuously distributed in the study site region of Tallaganda. These spiders are non-balloners which means the dispersal into their currently large range must have been over a long time. Biological characteristics include both species are of similar size, long lived and construct permanent burrow retreats (homes). Prey items include other invertebrates and small vertebrates that ends up in the funnel of death! Previous research looked at burrow detritus and lab experiments to look for specific prey preference. No definitive preference was found, so it was assumed the diet is based on what is in its microhabitat. These species both share common characters as referenced above, however have a marked variation in where they build their homes. A.sutherlandi burrows exclusively in soil, where H.cerberea is saproxylic meaning it makes its burrows in decomposing logs on the forest floor!

 

The study site Tallaganda, is shown in the above image. The pink areas are soils of metasedimentary segments dating back to the Ordovician period (490-443 Mya). The areas on the above image that are brown are dated from (443-354 Mya), making this landscape geologically stable. This region was subject to 100,000 year glacial interglacial cycles within the Pleistocene, with 20 glacial-interglacial periods in the last 2.4 Myr. As Tallaganda is within both A.sutherlandi and H.cerberea's wider distribution it implies they are not recent arrivals, therefore historical conditions have been the same for them. The alternate hypothesis is that both species show disparate phylogeographical patterns. This means there life histories and niches differences have played a role in there evolution which varies with each other.
The results were consistent with the alternate hypothesis that these two sympatric, niche differentiated species have different underlying population structures!

H.cerberea population structure suggest a recent radiation from a point source or recent colonization of Tallaganda. Low nucleotide diversity and high haplotype diversity is found in H.cerberea  which is consistent with this idea of recent rapid expansion. Either a small population survived in Tallaganda during the last glaciation and radiated, or the more likely idea that they recolonized from the Greater Diving Range (GDR). The GDR overlaps Tallaganda, which had individuals from the Monga forest in the GDR sequenced. The H.cerberea from Tallganda were shallowly nested (within the same clade, or close to) with the Mongo forest individuals meaning this is probably the area they recolonized from. This pattern is not found in any other saproxylic species studied within the Tallganda region, indicating that something about H.cerberea biology or association with its habitat is the causal factor. This pattern is proposed to be due to H.cerberea relying on wood to be present and decompose, so they can make burrows. But during glaciations the treelines are decreased substantially and no forest was left in the Tallaganda region. Therefore glaciation and environmental change will have a greater effect on H.cerberea and have an increased chance of causing local extinction.

The genetic analysis on H.cerberea has provided insight into how it has interacted in the Tallganda region, due to its more dynamic niche. A.sutherlandi however are indicative of long-term persistence within the Tallaganda region during glacial periods. H.cerberea had a phylogeny with a low variety of clades showing expansion from one source back into Tallaganda. A.sutherlandi however displayed a phylogeny which was more monophyletic, corresponding to a population that is distinct and has non-overlapping geographical ranges. Phylogeny of A.sutherlandi showing genetically and geographically distinct clades. This pattern is consistent with the idea that small areas of refugia were maintained during the glacial cycle. This means there was areas that the two main different soil types, in areas not covered or impacted by ice, allowing burrows and habitats to be built and sustained. This allows evolution and preference to the soil type of the refugia. These are also barriers to gene flow and is why there is several distinct genetic clades. I don't want to go into excess detail about soil type preference so I'll keep it short and sweet. Certain groups had a preference to either granite or metasediment soils based on what was found at the refugia. Post glacial, these groups began recolonizing the other soil type making paraphyletic groups (groups sourced from granitic soils) to monophyletic  metasediment clades. There is some degree of preference and structuring based on the underlying lithology. (Read the paper if you want a more in-depth look at the structuring of soil preference!).

Their are a few things that this can show us about evolution of invertebrates over time. One is that closely related but niche differentiated species like the funnel web spiders, can evolve differently as climate change affects their particular niche differently (no forest vs soil). This trivial difference of burrowing in wood vs soil shows that even the subtle and overlooked traits/ behaviours or niches can influence evolution drastically. Also as the funnel webs are a neglected taxon, it is important to realise how these taxon can show previously underestimated evolution history cues in certain environments, and on different organisms like invertebrates. I enjoyed this paper as it was on spiders :D. It also shows that phylogeography has a long way to go, but invertebrates are a possible solution to furthering our understanding of organisms distributed in space and time!

Thanks for reading! The link to the paper is posted below

Article: http://apps.webofknowledge.com.ezproxy.lincoln.ac.nz/full_record.do?product=UA&search_mode=GeneralSearch&qid=1&SID=Y13dKoo@2n8B2ldJAfC&page=1&doc=1