World species of the subgenus Oligonychus (Reckiella) Tuttle and Baker (Acari, Prostigmata, Tetranychidae), diagnostic keys, taxonomic notes, and a new species

Oligonychus Berlese (Acari, Prostigmata, Tetranychidae) is an agriculturally important and the largest genus of spider mites, comprised of 211 species (including new species), two subgenera, four species groups, and 11 species subgroups. The present study comprehensively addressed the morphotaxonomic-based identification of world species of the subgenus Reckiella Tuttle and Baker. Five diagnostic keys were developed for identifying Oligonychus (Reckiella) species belonging to five species subgroups: iseilemae, pritchardi, biharensis, gossypii, and exsiccator. Taxonomic notes are provided on intraspecific variations and some closely related Oligonychus (Reckiella) species representing six species complexes, viz. the afrasiaticus complex, the litchii complex, the pratensis complex, the plegas complex, the sacchari complex, and the tylus complex. One new spider mite species, Oligonychus bahaensis sp. nov., is described and illustrated from Grasses (Poaceae) under the subgenus Reckiella.


Methodology
All the published taxonomic literature of world Oligonychus species belonging to the subgenus Reckiella was collected through personal communication with acarologists in different countries, using various search engines/ websites as well as the websites of different acarological/entomological research journals.The published Oligonychus literature, i.e., original and subsequent descriptions of all species and regionally prepared diagnostic keys, was critically reviewed to develop a key for identifying Oligonychus (Reckiella) species.
Specimens of the new species (O.bahaensis sp.nov.) described in the present study were collected in 2019/2021 from grasses (Poaceae) in the Baha province of Saudi Arabia (SA).A sample of O. pratensis was also received from the country (USA) of its type locality, which was sent by our colleagues 18 at our request.Slide-mounted male and female specimens were observed under a BX51 phase-contrast microscope (Olympus, Tokyo, Japan), and the generic level identification was performed following the key of Bolland, et al. 3 .Whereas the subgenera, species groups, and subgroups were identified using the diagnostic key of Mushtaq et al. 5 , and species-level identification was performed following the key developed in the present study.The nomenclature of Lindquist 19 was followed for leg chaetotaxy and other terminologies, and Grandjean 20 and Grandjean 21 for body setae.Furthermore, following Mushtaq et al. 6 , different traits of the male aedeagus were measured to identify new species and for differentiation among closely related species belonging to the Reckiella (Fig. 1).All measurements for new species are provided in micrometers.First, the sizes are given for the holotype, followed by the range in parenthesis for paratypes.The collected mite specimens, including the holotype and paratypes, were deposited at the King Saud University Museum of Arthropods (Acarology section), Department of Plant Protection, College of Food and Agriculture Sciences, King Saud University (KSU), Riyadh, SA.
Figure 1.Morphological aedeagal parameters (H 1 : height of bent aedegal part, measured from the level when shaft dorsal margin just starting to bend upward (A either shaft dorsal margin straight, e.g. in Oligonychus afrasiaticus or B undulating, e.g. in O. oenotherae Smiley and Baker) to maximum height of knob dorsal margin, H 2 : height of knob stem, measured from the level when shaft dorsal margin just starting to bend upward to the level of measuring stem width, L 1 : length of shaft dorsal margin; L 2 : length of knob, W1: width of shaft, W2: width of knob stem, AL: length of knob anterior projection, PL: length of knob posterior projection, α1: angle formed between shaft axis and knob axis, and, α2: angle formed between shaft axis and bent part axis) that measured and used in the diagnostic keys for differentiation among some Oligonychus species of the subgenus Reckiella.
New species.Oligonychus bahaensis sp.nov.. Diagnosis (based on male and female)-Male: Empodium I with two claws, proximoventral claw subequal or slightly shorter than main dorsal claw; aedeagus turns dorsad with distinct knob, knob with narrowly rounded anterior projection and acute posterior projection, shaft axis forming strong acute angle with knob axis.Female: Tibia I with nine and tibia II with seven tactile setae; peritremes straight, with bulbous end.Opisthosoma medially with transverse striae, except longitudinal/irregular longitudinal or slightly oblique striae with/without forming V-shaped pattern in-between setae f 1 and f 2 , dorsal setae slender, serrate, not set on tubercles and longer than longitudinal distances to the bases of setae next in-line.
Male (Figs. 4 and 5; n = 1): Length of body (v 2 -h 2 ) 187; width (c3-c3) 140.Dorsum (Fig. 4A): Propodosoma medially with longitudinal striae; opisthosoma medially and posteriorly with typical transverse striae or irregular transverse striae, forming weak inverted V-pattern in-between setae f 1 and f 2 ; lateral idiosoma with longitudinal to oblique striae; all striae without lobes.Dorsal setae (except h 2 and h 3 ) slender, serrate, not set on tubercles and longer than longitudinal distances to the bases of setae next in-line.Lengths of dorsal setae:  www.nature.com/scientificreports/than anterior projection and stem width/height, knob length about 2.5 times or more than the stem-width and subequal or more than three times the stem-height; shaft axis forming strongly acute angle with knob axis and about right or weakly acute angle with axis of upturned part; shaft dorsal margin about 1.5 times or more longer than shaft maximum width and about twice or more longer than knob length; shaft dorsal margin forming an angle to shaft axis.Gnathosoma (Fig. 4E): Ventral infracapitulum with seta m 31.Palp spinneret suξ length 3 (3-4), width 2 (2-2.1),solenidion ω length 4 (4-5) (Fig. 4E).Stylophore rounded.Peritremes simple, straight and very slightly bulbous distally (similar to female).
Etymology: The specific epithet bahaensis was derived from the name of the province (Baha) of Saudi Arabia, from which the new species was collected.
Remarks: Oligonychus bahaensis.sp.nov.closely resembles O. pratensis (Banks), because both have female palp spinneret maximum two times longer than its width, only male empodium I with proximoventral claw, female/ male peritremes straight with normal length, not hooked distally, female tibia I with 9 and II with 7 tactile setae, and various similar aedeagal characteristics, e.g., aedeagus with the distinct terminal knob, knob with rounded anterior and acute posterior projections, the axis of aedeagal knob forming an acute angle with the shaft axis.However, the new species differs from O. pratensis by male aedeagus knob with posterior projection about subequal or slightly longer than anterior projection (vs.1.5 times longer), shaft dorsal margin maximum 2.5 the length of knob (vs.about three times longer) in O. pratensis (Fig. 6).Additionally, a high genetic divergence of 17.7% (ITS2) was observed between O. bahaensis sp.nov.(GenBank accession number: OR288175) and O. pratensis (GenBank accession number: OQ185450) 22 , aligned with the previous findings of interspecific divergences among various tetranychid species 7,23 .Although we comprehensively explored the spider mites' fauna from numerous localities/hosts in almost the whole of SA since 2018 5,7,22,24 , this new species O. bahaensis sp.nov. is observed to inhabit grasses in only one locality in Baha province.
Keys to the world Oligonychus species of the subgenus Reckiella (114 species, excluding O. comptus and five briefly described species).Key to species of the subgroup iseilemae (12 species).www.nature.com/scientificreports/

Discussion
A diagnostic key has been erected for distinguishing the subgenera, species groups, and species subgroups of the genus Oligonychus 5 .In the present study, five dichotomous keys are developed using male/female morphological characters to identify the world Oligonychus species belonging to five Reckiella subgroups (biharensis, exsiccator, gossypii, iseilemae, and pritchardi), except the subgroup comptus that having only one species 5 .However, five Oligonychus (Reckiella) species, namely, O. annonicus (McGregor), O. anneke, O. bruneri (Livschitz), O. stenoperitrematus (Ugarov and Nikolskii), and O. saccharoides could not be included in species-level keys due to the lack of required key morphological information in the published literature [25][26][27][28][29] .Among them, O. annonicus and O. stenoperitrematus did not even assign to any species group due to the unavailability of required female morphological characters.Also, O. bruneri was only assigned to the level of species group exsiccator 5 .The subgroup exsiccator is the most diverse grouping in Reckiella 5 .In this subgroup, we found some intraspecific variations in few species, namely, O. formosanus, O. indicus, O. modestus, O. pennisetum, and O. pratensis.In some specimens of O. pennisetum, the female striae pattern was observed as irregular instead of having transverse between setae e 1 -e 1

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. Also, the shape of male aedeagus in one reported population of O. indicus is totally different (knob dorsal margin concave, with distinct anterior projection) 4 from original or other described populations (knob dorsal margin straight, with almost no anterior projection) 16,30 .Likewise, one aedeagal illustration of O. formosanus looks somewhat different (shaft axis forming a nearly right angle to the axis of the bent part 31 , as compared to other reported populations 32,33 .In O. modestus, the knob dorsal margin shape was also variable from convex/rounded 4,11 to almost straight 34,35 .In O. pratensis, the shape of aedeagus (specifically aedeagal knob) was found to be variable among various populations, reported from different geographical localities 4,8,10,11,27,34,[36][37][38][39] .Indeed, the Chinese population of O. pratensis was detected as a cryptic species that even did not belong to the subgroup exsiccator 5,12 .Furthermore, Meyer 10 synonymized O. monsarrati Gutierrez with O. grypus and also agreed by the later authors 5 .are morphologically very closed to each other.However, some of these species are distinguished in the present study using some characteristics, e.g., the number of tactile/sensory setae on male tibia I, tactile setae behind to proximal duplex on male tarsus I, and the shape of male aedeagus.Still, some species (O.pratensis, O. sacchari, O. saccharinus) are difficult to differentiate using their available morphological information because key characters of aedeagus are variable in their expression among/within reported populations 4,8,[10][11][12]15,27,34,[36][37][38][39]41 . Therefore, we broght the specimens of O. pratensis from USA (the country of its type locality) and observed the key diagnostic character of male aedeagus to differentiate between O. pratensis (Fig. 6) and the new species O. bahaensis sp.nov.(Fig. 4C,D).
In conclusion, we developed five diagnostic keys based on the published morphotaxonomic literature to facilitate the identification of world Oligonychus (Reckiella) species.We suggest using molecular data with morphology to validate the identification of any newly described or closely related Oligonychus species.Integrative taxonomic approaches are crucial to resolving the issues of species complexes in the subgenus Reckiella.