Investigating the Phylogenies of Trapdoor Spiders
Abstract: Observing the morphological and molecular phylogenies of trapdoor spiders (genus: Myrmekiaphila) allowed the following guiding question to be answered: Does the phylogenetic trees for the morphological data and genetic data agree when classifying the spiders by genetalia? It is hypothesized that the morphological phylogeny (based on characteristics) and the molecular (based on DNA) phylogeny will not agree due to prezygotic barriers. In order to compare the two, a molecular phylogeny had to be generated using the application MEGA and genetic codes of different species of spiders that fell under the genus Myrmekiaphila. The results showed that the morphological data (provided by Bond and Platnick), derived from the genitalic characteristics of trapdoor spiders, had different evolutionary pathways than the molecular phylogeny. Different species of trapdoor spiders with dissimilar genitalia showed to overlap in many areas but no indication of convergence was present. The occurrence of the overlapping, distinct species was an indication of prezygotic mechanical isolation. The occurrence of overlapping similar morphological species is an indication of additional prezygotic barriers. The claim then supports the hypothesis since the two phylogenic trees show inconsistencies.
Investigation 2 followed the guiding question of: “Does the phylogenetic trees for the morphological data and genetic data agree when classifying the spiders by genetalia?” The trapdoor spider (genus: Myrmekiaphila) was the source of genetic and morphological data for this experiment. The spiders’ unique genetalia served as a source for distinguishing species, more specifically, for distinguishing males. “Males have two prominent derived genital structures…one of them apparently locks against a modified female structure mating clasper, while the other is inserted into the female atrium pedipalp” (Barrantes, 2013). Male spiders’ genitalia consist of pedipalps (smaller limb-like structures near face) with a bulbus end, and an embolus tip (similar to a stinger) that can be described as branched or unbranched. To aid with mating, males also have mating claspers that vary in size, color, and spinal abundance. “Myrmekiaphila species are found in a diverse set of habitats and ecoregions, ranging from the relatively high elevation temperate deciduous forests of the southern Appalachian Mountains to the more xeric, low elevation climates of central Texas” (Bond, 2007). The members of this genus display geographical overlap in much of Alabama and Florida as well as along the Appalachian Mountains. When studying phylogeny, one wants to assume that species are parsimonious, however “because of their spiders rapid evolution and species-specificity, their illustration is a common feature of taxonomic literature to discriminate closely-related species” (Yassin, 2013). In other words, the spiders have extremely similar morphologies so it is difficult to confirm common ancestries solely based on traits. This is why the investigation includes a molecular phylogenic tree, where one can create ancestry based off genetic information. Using the background information as reference has driven the hypothesis of: If the spiders were affected by any prezygotic barriers, then the morphological tree will not agree with the genetic tree because there are other factors at work than just morphologies and genetics.
Methods & Results:
Files containing a phylogenic morphological tree as well as genetic codes for trapdoor spiders were provided for this investigation. The morphological phylogeny (by Bond and Platnick) exhibited the relationships between trapdoor male spiders with a branched (double-pronged) or unbranched (mono-pronged) pedipalp embolus. In order to relate the morphologies to the genetic data, a molecular phylogeny had to be created. The application known as MEGA was used to insert genetic codes of all the species of Myrmekiaphila to be analyzed and organized. These species included M. coreyi, M. comstocki, M. tigris, M. howelli, M. millerae, M. fluviatilis, M. jenkinsi, M. foliata, M. neilyoungi, M. torreya, promyrmkiaphila, and aptostichus. The file containing the genetic code was already trimmed to the prioritized genes and the lab required that the “maximum likelihood tree” be set for construction. Using the DNA sequences of the trapdoor spiders in the “maximum likelihood tree” setting would allow MEGA to draw lineages of the most similar DNA. Other parameters set would include the bootstrapping model. This allows MEGA to randomly sample the DNA instead of assume linkage. Suggesting the Jukes-Cantor model and a 100 nucleotide count, the tree was generated and M. Apostichus was out grouped for reference. Creating the molecular phylogeny reveals the actual evolutionary path of the trapdoor spiders using the ties in DNA coding.
Figure 1. Molecular Phylogeny of Genus Myrmekiaphila. Made via MEGA
Comparing the morphological phylogeny to the molecular phylogeny, it is clear that the two do not agree when classifying the spiders by genetalia. The claim is as stated: due to the disagreement of the morphologic phylogeny and the molecular phylogeny, the hypothesis is supported. M. torreya, M. comstocki, and M. neilyoungi show a drastic difference on where they sit on the phylogeny. The same monophyletic groups are not reserved in the molecular phylogeny as they were in the morphological tree. Unlike the Bond and Platnick phylogeny, the molecular phylogeny shows that there is no plausible scenario where each genitalic morphology evolved at a single time. For example, M. coreyi and M. tigris were expressed as having unbranched genital bulbs. According to the morphological phylogeny, these two species should be descended along with other species that have unbranched genital bulbs. However, the molecular phylogeny debunks this use of parsimony and shows M. coreyi and M. tigris as sharing recent common ancestors with species characterized as having branched genital bulbs. This can also be displayed by looking at M. howelli, M millerae, and M. torreya. In the case of range over lap, looking specifically along the Appalachian Mountains M. foliata (unbranched) is overlapped with all branched species (M.neilyoungi, M. torreya, M. jenkinsi, and M. fluviatilis). This is an indication of a prezygotic barrier known as mechanical isolation where the unbranched and branched species coexist in the same areas but no divergence of the two species occurs. M. foliata’s unbranched genitalia does not mechanically match the same way a M. foliata’s branched genitalia does with a M. foliata female and therefore they are mechanically isolated. Even in the overlap of three species in the panhandle of Florida (M. correyi, M. torreya, and M. minuta). It is known that M. correyi is unbranched and M. torreya is branched; however the gentialic bulb is unspecified in M. minuta. It can still be expected that M. minuta will be dissimilar and prezygotically isolated because reinforcement may have already further differentiated all three species. Reinforcement could have additionally made behavioral and temporal isolation occur where the three species are now mating at different times or having different mating rituals. Trapdoor spiders that exhibit a branched or unbranched embolus that is inconsistent with other sister taxa can be best explained by convergent evolution. Convergent evolution explains that regardless of ancestry, similar characteristics may occur. Conclusively, it is supported through data, that the morphological and molecular phylogenies do not agree when classifying spiders by genetalia.
Works Cited & References
Barrantes, Gilbert, et al. “Functional Aspects of Genital Differences InLeucauge ArgyraandL. Mariana(Araneae: Tetragnathidae).” Journal of Arachnology, vol. 41, no. 1, 2013, pp. 59–69., doi:10.1636/b12-63.1.
Bond, Jason E., and Norman L. Patnick. “A Taxonomic Review of the Trapdoor Spider Genus Myrmekiaphila.” American Museum Novitiates, 12 Dec. 2007, pp. 1–10.
Nik Lupše, et al. “Coevolution of Female and Male Genital Components to Avoid Genital Size Mismatches in Sexually Dimorphic Spiders.” BMC Evolutionary Biology, BioMed Central, 17 Aug. 2016, bmcevolbiol.biomedcentral.com/articles/10.1186/s12862-016-0734-9
Yassin, Amir, and Virginie Orgogozo. “Coevolution between Male and Female Genitalia in the Drosophila Melanogaster Species Subgroup.” PLOS, vol. 8, no. 2, 25 Feb. 2013, pp. 1–12., journals.plos.org/plosone/article?id=10.1371/journal.pone.0057158.