Research Article |
Corresponding author: Sibylle Häggqvist ( sibylle.haggqvist@nrm.se ) Academic editor: Martin Hauser
© 2015 Sibylle Häggqvist, Sven Olof Ulefors, Fredrik Ronquist.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Häggqvist S, Ulefors SO, Ronquist F (2015) A new species group in Megaselia, the lucifrons group, with description of a new species (Diptera, Phoridae). ZooKeys 512: 89-108. https://doi.org/10.3897/zookeys.512.9494
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With 1,400 described species, Megaselia is one of the most species-rich genera in the animal kingdom, and at the same time one of the least studied. An important obstacle to taxonomic progress is the lack of knowledge concerning the phylogenetic structure within the genus. Classification of Megaselia at the level of subgenus is incomplete although Schmitz addressed several groups of species in a series of monographs published from 1956 to 1981. Another problem is the lack of molecular phylogenetic analyses to support morphology-based conclusions. As a contribution towards addressing these problems, we here circumscribe a previously unrecognized monophyletic lineage of Megaselia consisting of species similar to M. lucifrons. We base this taxonomic decision on morphological study of an extensive phorid material from Sweden, complemented by molecular analyses of select exemplars using two markers (COI and 28S). We name the clade the lucifrons species group, and show that it contains three distinct species. Our results also demonstrate that Megaselia subnitida Lundbeck, 1920, previously treated as a synonym of M. lucifrons Schmitz, 1918, is a separate species, and we remove it from synonymy. The third species in the group was previously unknown; we describe it here as M. albalucifrons sp. n.
Phoridae, new species, Megaselia albalucifrons sp. n., Megaselia lucifrons, Megaselia subnitida
The Phoridae (Diptera), or scuttle flies, are one of the most diverse families of flies. There are currently more than 4,000 described species (
It is unclear at this point whether Megaselia is monophyletic with respect to all of the remaining genera in the family, and the sheer size of the genus makes it difficult to study its phylogenetic structure. Indeed, the lack of knowledge concerning higher-level relationships within Megaselia is probably the most important impediment to further taxonomic progress on the genus. Although Schmitz indicated which groups within Megaselia he recognized, as published in Lindner’s “Die Fliegen” (1956–1981), a complete revision of the genus has never been completed and his work remains unfinished. Schmitz divided the genus into two subgenera, Megaselia and Aphiochaeta (Schmitz, 1926–1927), based on the pubescence of the anepisternum: it is naked in the former but hairy in the latter. The subgenera were then divided into divisions (“Abteilungen”) and sometimes further into series. The divisions were described in the first publication in this series on Megaselia (Schmitz, 1956), which were followed by revisions of each division, with keys to the species and morphological descriptions of all species. Unfortunately, the subgenus Megaselia with division VI to VIII was never published. For division VI, there is a key to the species but no descriptions of them.
During our work on phorid material from the Swedish Malaise Trap Project (SMTP), a large-scale inventory of the Swedish insect fauna (
Majority rule consensus tree from a Bayesian phylogenetic analysis based on combined COI and 28S D2 data, showing that the lata group, as originally circumscribed by us, consists of two distinct but unrelated lineages that are consistent with the two subgenera Megaselia and Aphiochaeta: the lata group sensu stricto and the lucifrons group. Numbers on branches are posterior probabilities (PP, in percent). PP values are not given for branches inside the terminal clusters corresponding to species. The scale bar represents nucleotide substitutions per site. Numbers within boxes (LAXXX) are specimen IDs and indicate which specimens were used in the analysis. Specimens marked with asterisks (*) represent morphospecies that were not supported as distinct by the molecular analysis.
In this paper, we focus on the latter group. We name this clade the lucifrons species-group and characterize it morphologically. Based on molecular and morphological data, we show that the group contains three species: M. lucifrons; M. subnitida Lundbeck, 1920; and M. albalucifrons sp. n., which is described here (Fig.
The initial morphological studies were based on a large phorid material, mostly coming from the SMTP. The SMTP sampled the Swedish insect fauna using 75 Malaise traps run for three consecutive years (2003–2006) at 54 different sites, selected to represent a wide range of habitats (
For the initial molecular analysis, we used 101 specimens from species representing the lata group, sensu lato, with M. impolluta and M. hibernans as outgroups. One of the lineages identified in this analysis, called the lucifrons group here, was selected for more detailed study. Specifically, 46 specimens of the lucifrons group were sequenced and 14 of these specimens were dissected. Based on the initial molecular analysis, we identified Megaselia lata (Wood, 1910) and M. hibernans Schmitz, 1934 as suitable outgroups for the analyses of the lucifrons group.
Repositories of studied specimens and corresponding abbreviations are: Swedish Museum of Natural History (NHRS); Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany (ZFMK); Natural History Museum of Denmark (ZMUC) and Natural History Museum of Los Angeles County, Los Angeles, USA (LACM). The holotype and paratypes of Megaselia albalucifrons are deposited in the collection of the Swedish Museum of Natural History.
Specimens were examined under a stereomicroscope with magnification up to 60×. Two specimens from each of the three species were dissected in order to examine different parts of the male genitalia (epandrium, hypandrium and phallus) more closely under a compound microscope with 400× magnification. Paper and pencil drawings were prepared using a camera lucida fitted to a compound microscope. The drawings were digitized and redrawn in Adobe Illustrator. Terminology of the male genitalia follows
Genomic DNA was extracted using QIAGEN mini kit or GenMole robot. Two different DNA fragments were amplified and sequenced: the mitochondrial cytochrome c oxidase subunit I (COI) and the D2 variable expansion region of nuclear 28S rRNA (28S). The oligonucleotide primers used are LCO1490 and HCO2198 as listed in
Amplified products were checked on a mini-agarose-gel and purified using EXO1 and FastAP (EXOFAP). DNA sequencing was performed using the same primers as in the PCR reactions and the BigDyeTM Terminator ver. 3.1 Cycle Sequencing Kit (Applied Biosystems). Sequencing reactions were purified using the DyeEx 96 kit (QUIAGEN) and the samples were sequenced on an ABI Prism 3100 Genetic Analyzer (Applied Biosystems).
Sequence electropherograms were read and edited using Geneious Pro 5.3.4 (Biomatters Ltd.). Alignments for both genes were performed inside Geneious Pro 5.3.4 using the Consensus align option. All sequences were deposited in GenBank (Suppl. material
To study seasonal patterns in adult activity, the occurrence data for Megaselia lucifrons were recorded from the 24 individuals that were used in this study and an additional 268 SMTP specimens identified as M. lucifrons by SOU. The collection dates (Malaise trap emptying dates) were pooled into 10.5 days long periods. We did not have enough data for the other two species to permit similar analyses.
Genetic distances and numbers of parsimony informative characters were obtained using MEGA version 6 (
Species circumscription was further analyzed using the program BP&P (
To this date, we have identified approximately 65 species groups within Megaselia based on morphological characters. Some of these groups, such as Kryophile, Crassitarsale and Simplicitarsale have already been described by Schmitz and others, but most are new, even though they sometimes form subgroups of more familiar groups, such as the pulicaria complex (e.g.
The lata group, as originally circumscribed by us (sensu lato), is characterized morphologically by the large and spinose labellum, the long costal vein that is about half the length of the wing, the long first costal sector, the short costal cilia and the strong proctiger hair. In the phallus, the base of the left arm projection is clearly sclerotized. These characters are not unique to the lata group, except possibly the sclerotized base of the left arm of the phallus, but the combination is unique and not found in other members of Megaselia.
Defined based on this suite of morphological characters, the lata group diagnosis cuts across the subgenera Megaselia and Aphiochaeta recognized by
Further studies of the lata suite of morphological characters indicated that there is a difference between the lata group, sensu stricto, and the lucifrons group in the left arm of the phallus. The left arm projection of the phallus is found in species of the M. lucifrons group but not in species of the M. lata group. Males of the lucifrons group (except M. subnitida) also have a light patch on the hind femur, which is absent in males of the lata group, sensu stricto. If the absence in M. subnitida is secondary, this may also be a good character distinguishing the groups.
In total, we analyzed phylogenetic relationships among 48 specimens belonging to the lucifrons group or one of the two outgroup species, M. lata and M. hibernans. Across the entire dataset, the COI alignment (658 bp) had 116 variable nucleotide positions, 52 of which were parsimony informative. The 28S alignment (518 bp) contained 21 variable nucleotide positions, 10 of which were parsimony informative.
In the Bayesian phylogenetic analysis of the combined data, the lucifrons group was monophyletic with respect to the included outgroups with a posterior probability (PP) of 100% (Fig.
Majority rule consensus tree from a Bayesian analysis of lucifrons group relationships based on COI and 28S D2 data. Megaselia lata and M. hibernans were used as outgroups. Numbers on branches are posterior probabilities (PP, in percent). PP values are not given for branches inside the terminal clusters corresponding to species. The scale bar represents nucleotide substitutions per site.
Species monophyly was well supported (PP 98–100%). Intraspecific genetic distances in COI were 1–2%, while interspecific differences varied from 14 to 21%. The analyses also clearly indicated that M. lucifrons and M. subnitida are more closely related to each other than to M. albalucifrons. The results from the BP&P analysis indicate that the three species of the lucifrons group are separate species, with a posterior probability of 0.93 to 0.94 for delimiting five species (two outgroup species in addition to the three species of the lucifrons group). The results from the separate COI analysis were very similar to the results from the combined analysis, whereas the separate 28S analysis results were less well resolved but not in conflict with the COI results.
The morphometric analyses revealed that M. subnitida is, on average, a larger species than the other two, with longer wings relative to the body length (Table
Measurements of body length, frons width and height, wing length and lengths of the costal sectors in mm for males of the three species. Costal sector ratio is given as the ratio of all three costal sectors divided by costal sector 3. Numbers in parentheses behind species name indicate the number of individuals measured.
Species | M. albalucifrons (6) | M. lucifrons (27) | M. subnitida (18) |
---|---|---|---|
Body length | 1.65 ± 0.09 | 1.58 ± 0.08 | 1.78 ± 0.12 |
Frontal width/height | 1.05 ± 0.06 | 1.08 ± 0.04 | 1.16 ± 0.06 |
Wing length | 1.41 ± 0.11 | 1.38 ± 0.09 | 1.66 ± 0.07 |
Costal lenth | 0.61 ± 0.06 | 0.62 ± 0.03 | 0.75 ± 0.06 |
Costal sector 1 | 0.40 ± 0.04 | 0.43 ± 0.03 | 0.51 ± 0.06 |
Costal sector 2 | 0.13 ± 0.03 | 0.11 ± 0.02 | 0.16 ± 0.02 |
Costal sector 3 | 0.09 ± 0.01 | 0.07 ± 0.01 | 0.09 ± 0.01 |
Costal sector ratio | 4.44 : 1.44 : 1 | 6.14 : 1.57 : 1 | 5.67 : 1.78 : 1 |
The male genitalia show a number of species specific differences. The entire epandrium in M. subnitida is distinctly expanded ventrally (Fig.
Both M. subnitida and M. lucifrons are common and widely distributed in Sweden (Fig.
So far, the data are too sparse to support strong assertions with respect to habitat preferences of lucifrons group species, although many specimens examined were collected in Malaise traps placed in wet or forested habitats. For M. lucifrons it appears that there are two generations per year (Fig.
To encourage additional taxonomic and systematic investigations of Megaselia, we believe it is important to study its phylogenetic structure so that it can be broken down into smaller, natural groups suitable for more detailed study. However, it will be some time before a clear picture of the deep phylogenetic structure of the entire genus will emerge, whose sufficiently broad sampling of lineages supports a stable subgeneric classification of the genus. Meanwhile, we think it will be productive to successively carve out and name monophyletic clades as informal species groups, which can later be revised in scope or renamed as knowledge increases. It is in this spirit we here define the lucifrons group.
Species of the lucifrons group have a wide and spinose labellum, which is quite uncommon for species of Megaselia of this small size (1.5–2.0 mm). The labellum is wide, more specifically, at least as wide as half of the frons (distance from middle of frons to dorsal frontalorbital seta). Costal ciliae are short (i.e. shorter than the third costal sector) which is characteristic for the group. The proctiger hair on the male genitalia in all three species of the lucifrons group is strong compared with hairs on the cercus (Fig.
Disney formally synonymized M. lucifrons and M. subnitida in 1988 based on external morphological characters but without close examination of the male genitalia (
Separation of species in Megaselia has been based primarily on differences in external morphological characters. Our results indicate that even slight variations in morphology, such as wing proportions and body size, can be good indicators for separating species. It is also clear that there are more morphological characters in the male genitalia than have been used previously for species identification. One reason that male genitalia characters remain poorly explored by phorid taxonomists is that many of the relevant characters are not possible to study when using the generally accepted slide mounting technique described by
Substantial work remains to be completed before we can understand phylogenetic relationships within the genus Megaselia and address its relationships with other genera in the family. In the near future, we hope to contribute more broadly to resolving these problems with molecular methods (Häggqvist et al. in prep.). Until then, we need to develop methods that allow us to productively contribute small pieces to this puzzle. Our results argue for attempts to define and circumscribe monophyletic lineages of Megaselia based on morphological and molecular data as one of these methods.
Differential diagnosis. Species of this group share the following features with the lata group (sensu stricto): The labellum is wide and spinose. When it is not expanded, such that pseudotracheae are visible, its width is between 130 and 170% of the half-width of the frons. The costal vein is almost half of the total wing length and the first costal sector is larger than the second and third together (Fig.
Unlike the lata group (sensu stricto), the hind femur is lighter proximally in males of Megaselia lucifrons and M. albalucifrons. It is unclear whether this is a ground-plan feature of the group, since the light femoral patch is lacking in males of M. subnitida. However, the fact that the patch is present in the closely related M. lucifrons suggests that the lack in M. subnitida is secondary.
The anepisternum in all species of the group is pilose but without the setae found in some members of the subgenus Aphiochaeta.
Remarks. Three species are known in the group. They have been recorded from several European countries and likely have a Western Palaearctic or Palaearctic distribution. Since the diversity of Megaselia is poorly known in general, it seems likely that further species belonging to the group will be discovered. Females belonging to the lucifrons group can be recognized by the combination of a large labellum, characters in the wings and the pilose anepisternum, but we are not yet able to confidently identify them to species. The morphological diagnoses below refer, therefore, exclusively to males. The biology of the group as well as the relationships to other lineages in Megaselia is unknown (but see Fig.
1 | Hind femur uniformly brown | Megaselia subnitida |
– | Hind femur with triangular-shaped lighter patch at the base (Fig. |
2 |
2 | Left lobe of the hypandrium broad (Fig. |
M. lucifrons |
– | Left lobe of hypandrium narrow (Fig. |
M. albalucifrons sp. n. |
Aphiochaeta lucifrons Schmitz, 1918
Lectotype: male, pinned, Netherlands, Baaksem, Holl. Limbg. 11. VIII. 1915, coll. Schmitz (ZFMK).
Suppl. material
Males are easily distinguished from M. subnitida by the triangular-shaped lighter patch on the base of the hind femur. M. lucifrons also has a shiny frons (hence its name), whereas the frons is matte in M. subnitida. Males can be distinguished from M. albalucifrons by the broader left lobe of the hypandrium (Fig.
M. lucifrons is widely distributed in Sweden and has been found in all 29 SMTP traps from which we have studied material. The species is also reported from many other European countries (e. g.
Material from the SMTP shows that M. lucifrons likely has two generations per year (Fig.
Aphiochaeta subnitida Lundbeck, 1920
Megaselia subnitida Schmitz, 1927
Megaselia lucifrons Disney, 1988
Holotype: male, slide mounted, Denmark, Holte, Suserup Skov at Sorø, 6. VIII. 1918 (leg. Th. Mortensen) (ZMUC).
Suppl. material
Males of M. subnitida are distinguished from males of the other two species in the group by the uniformly colored hind femur and the broader hypandrial left lobe (Fig.
M. subnitida is widely distributed in Sweden, and is usually found together with M. lucifrons (Fig.
M. subnitida appears in different habitats, often in or in proximity to a forest, from May to September in the SMTP material (Suppl. material
In Disney’s key (1989), M. subnitida runs to couplet 104, together with M. lucifrons.
Holotype male, in alcohol, (LA214) SWEDEN, Sm, Älmhults kommun, Stenbrohult, Djäknabygds bokbacke, Heath with old beeches. N56°36.548' E14°11.583' (=TrapID 24) 30.iv.-31-v.2005 (=coll. event ID 1672) Leg. Swedish Malaise Trap Project (Swedish Museum of Natural History).
Paratypes, 4 males, same as holotype (LA207, LA212, LA215, LA216).
The habitus (Fig.
Male: Frons dark brown and shiny with about 70 hairs, about as broad as long. Postpedicels brown. Palps light yellow with 6–8 setae. Labellum broad, light yellow as palps, densely covered with spinules. Anepisternum with 10–18 hairs. Thorax brown. Three notopleural setae, without cleft in front of them. Scutellum with anterior pair of hairs and posterior pair of setae. Abdominal tergites brown with hairs distally and laterally, especially T6 with long, strong hairs at the rear margin. Venter brown with hairs on segment 3–6. Hypopygium with brown epandrium with fine hairs on both sides, but not dorsally, and longer hairs at the lower margin (Fig.
The name is derived from the Latin word “albus” (white, pale), referring to the left lobe of the hypandrium, which is pale in comparison to that of M. lucifrons.
M. albalucifrons is found in Southern and central Sweden (Fig.
M. albalucifrons runs to couplet 104 in Disney’s key (
The study was financed by a grant (dha 158/09 1.4) to FR and SH from the Swedish Taxonomy Initiative. We are grateful to Dr. Johannes Bergsten for valuable advice regarding the molecular analyses, Dr. Chi Zhang for help with the BP&P analyses, Laura Noack for help with the Latin language and Dr. Kevin Holston for help with the English language. We would also like to thank Torbjørn Ekrem, Torbjörn Kronestedt, Birgitta Tullberg and two anonymous reviewers for constructive comments that greatly improved the manuscript.
Data type: species data
Explanation note: All specimens, except LA208 and LA209, which were caught by hand, come from the Swedish Malaise Trap Project (SMTP). All specimens are deposited in the collection of the Swedish Museum of Natural History.
Figure S1. Majority rule consensus tree from a Bayesian analysis of lucifrons group relationships based on COI data
Data type: molecular data
Explanation note: Megaselia lata and M. hibernans were used as outgroups. Numbers on branches are posterior probabilities (PP, in percent). PP values are not given for branches inside the terminal clusters corresponding to species. The scale bar represents nucleotide substitutions per site.
Figure S2. Majority rule consensus tree from a Bayesian analysis of lucifrons group relationships based on 28S D2 data
Data type: molecular data
Explanation note: M. hibernans were used as outgroups. Numbers on branches are posterior probabilities (PP, in percent). PP values are not given for branches inside the terminal clusters corresponding to species. The scale bar represents nucleotide substitutions per site.