Intraspecific variation in Gyrodactylus mediotorus and G. crysoleucas (Gyrodactylidae) from Nearctic shiners (Leuciscidae): evidence for ongoing speciation, host-switching, and parasite translocation Author Rahmouni, Chahrazed Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic Author Seifertová, Mária Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic Author Bean, Megan G. Texas Parks and Wildlife Department, 5103 Junction Highway, Mountain Home, TX, 78058, USA Author Šimková, Andrea Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic text Parasite 2024 Paris, France 2024-06-11 31 29 1 14 http://dx.doi.org/10.1051/parasite/2024023 journal article 10.1051/parasite/2024023 1776-1042 PMC11166112 38860920 12524595 A2F025CD-7379-4E84-921B-AC565CD1EAC8 Gyrodactylus mediotorus King, Marcogliese, Forest, McLaughlin & Bentzen, 2013 ( Figures 2A–2D ) Previous records: spottail shiner, N. hudsonius (type-host), Îles de la Paix , St. Louis Lake (type-locality) and Îles Vert, St. Lawrence Lake , both in Quebec , Canada [ 48 ]; weed shiner, N. texanus , Lake Onalaska , Upper Mississippi River , Wisconsin , USA [ 56 ]. Present study: blacktail shiner, C. venusta , West Mud Creek, Neches River ( Figure 2A–2B ), and sand shiner, N. cf. stramineus ( Guadalupe River), Guadalupe River ( Figure 2C–2D ), both in Texas , USA . Site of infection: fins Prevalence and intensity of infection: for C. venusta , 5.9%, two infected hosts out of 34 investigated, a single Gyrodactylus specimen on each infected host. For N. cf. stramineus ( Guadalupe River), 14.3%, two infected hosts out of 14 investigated, from 1 to 3 Gyrodactylus specimen per infected host. Voucher: IPCAS M- 794/1-2 Host GenBank accession numbers: cyt- b : PP314044– PP314045 for C. venusta , PP 314046 for N. stramineus Parasite GenBank accession numbers: 18S rDNA: PP309997; ITS : PP309999 Morphology: Haptor subcircular, anchor base with folds, tips curved outward, total length 59.4 (55.9–62.6; n = 8); shaft slightly bowed, length 42.9 (40–45; n = 8); point curved and elongate, length 21.5 (20.3–22.6; n = 8); root long, length 20.3 (18.3–21.6; n = 8). Ventral bar with prominent blunt lateral processes extending out of bar, total length 33.5 (30.7– 36.9; n = 8), total width 22.4 (20.7–24.8; n = 8), lateral processes length 7.8 (5.5–9.5; n = 8), distance between tips 26.1 (23–29.5; n = 8), median part with a noticeable knob, width 5.5 (4.3–6.9; n = 8), membrane (shield) rectangular with fine longitudinal striations, extending almost 1/2 of length of anchor shaft, length 18.1 (15.6–20.4; n = 8), width at the insertion 15.3 (12.3–17.6; n = 8). Dorsal bar straight with projections near each end, attenuated ends inserted into terminal plates, total length 23.7 (21–26.2; n = 8), width at midpoint 2.6 (1.8–2.9; n = 8). Marginal hooks total length 35.6 (31–38.8; n = 8); sickle foot moderate with downward globose heel, prominent triangular straightforward toe, conspicuous shelf; sickle proper almost as thick as toe base, shaft length 4.9 (4.4–5.4; n = 8); sickle length to shaft attachment 3.2 (2.8–3.7; n = 8); sickle proximal width 3.1 (2.6–3.5; n = 8); sickle distal width 3.4 (2.8–4; n = 8) ; point relatively thin and slightly curved, length 1.3 (1.1–1.7; n = 8); filament loop extending about 1/3 of handle length, length 8.5 (7.3–9.7; n = 8); handle ending in a noticeable posterior filament, length 30.1 (26.2–34; n = 8). MCO observed in a single Gyrodactylus specimen from N. stramineus cf. ( Guadalupe River) with a single apical prominent spine and a single row of seven spinelets. Size and shape of the sclerotized structures of G. mediotorus specimens from southeast populations of each of C. venusta and N. cf. stramineus ( Guadalupe River) overlapped. Compared to the type-material [ 48 ], noticeable intraspecific variability was observed, mainly in terms of the (i) shorter hamuli (55.9–62.6 µm in this study vs . 65.7– 69.7 µm in the original description), and (ii) the shorter ventral bar (30.7–36.9 µm in this study vs . 36.4–41.3 µm in the original description). Although well visible on the photographs, King et al. [ 48 ] did not mention the presence of a knob in the median part of the ventral bar or a prominent filament attachment posteriorly to the handle of the marginal hooks. The median knob of the ventral bar was later emphasized by Leis et al. [ 56 ] when reporting a variant of G. mediotorus on N. texanus , whereas the additional filament marking the posterior end of the marginal hooks was not highlighted. The specimens of G. mediotorus studied herein can be compared to the so-farunknown Gyrodactylus sp. “ C. venusta ” collected recently in Mississippi [ 82 ] in having a similar haptoral morphotype, but mainly because of the presence of the ventral bar knob and the filament of the marginal hooks. Yet, considerable variation in the size of the ventral bar is observed (30.7–36.9 µm in this study vs . 20.1 µm in [ 82 ]). Likewise, our specimens of G. mediotorus possessed a longer ventral bar than G. mediotorus from N. texanus (30.7–36.9 µm in this study vs . 22 µm in [ 56 ]).