The genome sequence of the dumpy grass hoverfly, Melanostoma mellinum (Linnaeus, 1758)

We present a genome assembly from an individual male Melanostoma mellinum (the dumpy grass hoverfly; Arthropoda; Insecta; Diptera; Syriphidae). The genome sequence is 731 megabases in span. The majority of the assembly (99.67%) is scaffolded into five chromosomal pseudomolecules, with the X and Y sex chromosomes assembled. The complete mitochondrial genome was also assembled and is 16.1 kilobases in length.


Background
Melanostoma mellinum is a small hoverfly with yellow and black markings, perhaps using this aposematic colourway to loosely mimic wasps and so gain protection from predators (Ball & Morris, 2015;van Veen, 2010). These hoverflies have dark faces and dark scutellums. Female Melanostoma species have distinctive inverted yellow triangle abdominal markings and can be separated from the other common species M. scalare by the narrow dust spots on the otherwise shining frons (Ball & Morris, 2015). Male M. mellinum ( Figure 1) have square yellow abdominal markings with tergite 2 and 3 being as long as wide, while M. scalare males have longer tergites (Ball & Morris, 2015;van Veen, 2010). Colour and morphological variation is high and M. mellinum may comprise a species complex (Haarto & Ståhls, 2014). M. mellinum is an abundant grassland species in the UK and can be found in greatest numbers during summer, feeding on the pollen of wind pollinated flowers such as grasses, plantain and sedges (Ball & Morris, 2015). The larvae of this species feed on aphids in the leaf litter and ground layer and are incredibly generalist predators, preying on at least 32 different aphid species (Dziock, 2005) and gall forming Psyllidae (Hodkinson & Flint, 1971). Like some other generalist hoverflies, M. mellinum adults display seasonal migratory behaviour (Aubert et al., 1976;Gatter & Schmid, 1990). This species seems to be heavily affected by the Entomophora fungal pathogen (Jensen & Eilenberg, 2001) and dead M. mellinum individuals can be often found hanging beneath flower heads with fungal spores exuding (WH, personal observation; Figure 1). Migration in this species may function to reduce the negative effects of this fungus on the population by allowing escape from contaminated habitats and by culling infected animals, both phenomena seen in other migrants (see Satterfield et al. (2018)). This is the first production of a high quality M. mellinum genome and we believe that the sequence described here, generated as part of the Darwin Tree of Life project, will further aid understanding of the biology and ecology of this hoverfly.

Genome sequence report
The genome was sequenced from a single male M. mellinum collected from Wytham Great Wood, Oxfordshire (Biological vice-county: Berkshire), UK (latitude 51.769, longitude -1.330) ( Figure 1). A total of 26-fold coverage in Pacific Biosciences single-molecule long reads and 55-fold coverage in 10X Genomics read clouds were generated. Primary assembly contigs were scaffolded with chromosome conformation Hi-C data. Manual assembly curation corrected 332 missing/misjoins and removed 35 haplotypic duplications, reducing the assembly size by 2.84% and the scaffold number by 74.75%, and increasing the scaffold N50 by 395.90%.  The final assembly has a total length of 731 Mb in 76 sequence scaffolds with a scaffold N50 of 235 Mb (Table 1). The majority, 99.67%, of the assembly sequence was assigned to five chromosomal-level scaffolds, representing three autosomes (numbered by sequence length), and the X and Y sex chromosomes (Figure 2- Figure 5; Table 2 Illumina HiSeq X (10X) instruments. Hi-C data were generated from abdomen tissue of idMelMell2 using the Arima        The data note article written by Hawkes and Wotton describes the sequencing and subsequent assembly of the Melanostoma mellinum genome using long-read sequencing on a PacBio platform.
In my estimation, the authors succeeded in generating a well-documented and accessible dataset. I only have minor criticism regarding the data note's writing style and presentation.

Background
Melanostoma mellinum is a small hoverfly with yellow and black markings, perhaps using this aposematic colourway to loosely mimic wasps and so gain protection from predators ○ This sentence is odd for the first sentence of a background.
(1) Many hoverflies are assumed to mimic wasps not just M. mellinum.
(2) The word 'perhaps' suggests that this is rather a musing instead of a factual description. I would (1) prefer to start describing a trait that stands out for this species in particular and (2) move factual descriptions towards the start of the paragraph and include assumptions towards the end of the paragraph.

Migration in this species may function to reduce
○ "Migration in this species might reduce" would be a better sentence in my opinion.
This is the first production of a high quality M. mellinumgenome and we believe that the sequence described here, generated as part of the Darwin Tree of Life project, will further aid understanding of the biology and ecology of this hoverfly. ○ I encourage the authors to elaborate on this. Do you think your dataset will assist researchers in answering specific research questions regarding the biology and ecology of this species? Do you expect (or even observe) duplication or loss events for certain genes or genes of certain gene families in comparison to other closely related taxa?
The public availability of new molecular biologicals dataset is in itself valuable, but I would wish for data of Arthropods before (primarily Hymenoptera, but I have also worked with sequencing data from hover flies). More recently, my professional interests shifted towards biomedical informatics. I analyze the Illumina sequencing data of tumor biopsies from human patients. I have theoretical, but no practical experience with long-read sequencing data and while I'm experienced in bioinformatics, I cannot evaluate the laboratory protocols used.
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