Phylogeny Overview

Phylogenies of insect species used to be solely reliant on morphological characters before the advent of molecular analysis, which in many cases limited the ability of researchers to confidently place species, genera, and families in relation to each other. Recent advances in DNA sequencing and molecular phylogeny computations have opened up the door to studies on relationships between/within groups previously considered cryptic or of indeterminate relationship. The following will be a review of studies focused on phylogenies within Strauzia and in the surrounding subfamily Trypetinae.

For the genus Strauzia itself, the most comprehensive study on phylogenetics has been that of Hippee et al., 2020. This phylogeny used the 3RAD technique where whole genomic data was split up using a collection of restriction enzymes, and then these small bits are pooled and sequenced in 150 base pair chunks. Raw sequenced DNA data was then analyzed across all of the samples to find SNPs (single nucleotide polymorphisms, changes in one base pair across different species), with the end data sets totaling around 15,000 SNPs analyzed for the phylogeny.

(Figure 3 from Hippee et al., 2020)

The resulting phylogeny revealed the relationships between species, and proved that species that had previously been interpreted as the same were actually not each other’s closest relatives. Each branch of the phylogeny is a single specimen, which is why there are so many words on the diagram; the stars indicate species of Strauzia that share the same host plant. Below is a simplified version that makes seeing relationships between species in the genus Strauzia easier to interpret.

(Simplified phylogenetic tree created in Mesquite by me, stars added to match above figure; White = H. grosseserratus, Black = H. tuberosus. Horizontal distances between nodes equalized and not scaled based on evolutionary/genetic distance.)

[iNaturalist Photo 85902198, (c) akilee, some rights reserved (CC-BY)]. A male Strauzia fly from Massachusetts.

One of the most interesting patterns that emerge from this phylogeny is that the species of Strauzia that use the same host plant species are not actually each other’s closest relatives. This is especially notable because in the past, species on the same plant have been hard to differentiate from each other. For instance, S. longipennis and S. vittigera have been considered variants of a single species for at least a hundred years, feeding on the same plant, but this recent study showed a large relative genetic difference between the two with species being placed in between. Also, S. vittigera has been thought to be a polyphagous species (eating multiple species of plants), but this phylogeny implies that the different groups of S. vittigera flies that go to different host plants are genetically separated from each other. This could mean this group too should be split into distinct species, but only future studies will be able to further this idea.

As for the closest relatives of Strauzia, it is placed within the subfamily Trypetinae and the tribe Trypetini (Norrbom, 2004). However, the relationships between the genera of Trypetini are relatively obscure. The main studies elucidating molecular phylogenetic relationships in this group have been performed by Ho-Yeon Han over the past two decades. This research may have a location bias, as only the Eastern US, Korea, Japan, and Switzerland were the localities of flies sampled for the following studies despite Trypetini’s presence across the temperate regions of the northern hemisphere (Norrbom, 2004).

Itosigo bellus flies mating, from Choi, 2014.

(Figure 2 from Han H-Y, 2000)

The first phylogenetic study of Trypetini using genetic data was published in Han H-Y, 2000. This study was based on the 16S gene, which is found on the mitochondrial genome and is often used for phylogenetic studies due to its relatively slow rate of evolution (Weisburg et al., 1991). The resulting phylogeny placed the genus Strauzia as sister to Itosigo bellus. Itosigo bellus is found in Japan and Korea, and no host plants are known (Norrbom et al., 1999). However, this result is tenuous, as the bootstrap value is about 50% at the node connecting these two groups, meaning that only in half of the calculations was this grouping supported.

(Figure 2 from Han H-Y, 2012)

A more recent study of Trypetini was performed by Ho-Yeon Han in 2012, using the same 16S gene and analytical techniques as the previous one, but with an increased 9 species sampled. In this phylogeny, Strauzia is removed from close association of Itosigo bellus, and instead grouped with Vidalia armifrons. No pictures of this species exist in literature, but a congener Vidalia accola is more common. V. accola’s range goes from Northern India across to Southern Japan, stretching down to Myanmar (Norrbom et al., 1999). Members of Vidalia breed in the fruits of Schefflera (Mir and Mir, 2015), which differs from the life history of Strauzia. One interesting character of note is the inflated frontal bristles of male V. accola, which is a similar trait to male Strauzia flies.

(Isolated from Figure 1 of Han H-Y, 2012) Vidalia accola, male on left and female on right.

One interesting pattern in North America is that all of the close relatives of Strauzia in Trypetina (a subtribe of Trypetini) do not use fruits as their larval host site as many Trypetine flies do in other regions of the world. T. flaveola is a generalist leaf-miner on many families of plants, which is relatively rare for Tephritidae (Frick, 1974). Leaf-mining is shared with the third genus of Nearctic Trypetina, Euleia, which is restricted to carrots/parsnip leaves (Tauber and Toschi, 1964). No studies have been done on why this distribution of Trypetina life cycles exists in North America.

[iNaturalist Photo 29836360, (c) skitterbug, some rights reserved (CC-BY)]. A female Trypeta flaveola from Georgia.

Works Cited

Choi D-S (2014). An introduction to diagnostic method for Bactrocera species [Conference presentation]. IPPC Secretariat 2014. Rome, Italy.

Frick KE (1971). The Biology of Trypeta angustigena Foote in Central Coastal California – Host Plants and Notes (Diptera: Tephritidae). J Wash Acad Sci. 61(1): 20-24.

Han H-Y (2000). Molecular phylogenetic study of the tribe Trypetini (Diptera: Tephritidae), using mitochondrial 16S ribosomal DNA sequences. Biochem Syst Ecol. 28: 501-513.

Han H-Y (2012). Pseudovidalia Han (Diptera: Tephritidae: Trypetini), a new genus from East Asia proposed based on morphological and molecular data. J Asia-Pac Entomol. 15: 419-425. https://doi.org/10.1016/k.aspen.2012.05.014

Hippee AC, Beer MA, Bagley RK, et al. (2020). Host shifting and host sharing in a genus of specialist flies diversifying alongside their sunflower hosts. J Evol Biol. 1–16. https://doi.org/10.1111/jeb.13740

Maddison WP and Maddison DR (2019). Mesquite: a modular system for evolutionary analysis. Version 3.61. http://www.mesquiteproject.org

Mir SH and Mir GM (2015). First record of Vidalia accola (Hardy) (Diptera: Tephritidae) with a key to species from Northwestern India. Entomol News. 125(4): 275-280. https://bioone.org/journals/entomological-news/volume-125/issue-4/021.125.0408

Norrbom AL, Carroll LE, Thompson FC, White I, and Freidberg A (1999). "Systematic database of names." Myia. 9:65–299.

Norrbom, AL (2004). Fruit fly (Tephritidae) names and bibliography database. http://www.sel.barc.usda.gov/ (Version Nov. 2004).

Tauber MJ and Toschi CA (1965). Bionomics of Euleia Fratria (Loew) (Diptera: Tephritidae). Can J Zool. 43: 369-379.

Weisburg WG, Barns SM, Pelletier DA, Lane DJ (1991). 16S ribosomal DNA amplification for phylogenetic study. J. Bacteriol. 173(2): 697-703. https://doi.org/10.1128/jb.173.2.697-703.1991

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