Introduction

The endemic, black-polled yellowthroat (Geothlypis speciosa) is a New World warbler that resides in freshwater marshes, known just from four areas in central Mexico, in the states of Guanajuato, Mexico and Michoacan (Pérez-Arteaga et al., 2018). The species is listed as globally vulnerable by BirdLife International (2023), and under the Mexican legislation, at risk of extinction (DOF, 2010). The male of this parulid has a broad black mask on the face, a black forecrown, but the mask does not have a pale upper border as in other yellowthroats. The female is olive, brown on the dorsum and yellow on the venter, with dusky olive flanks (Antolin & Ghalambor, 2020). Two subspecies, G. s. limnatis and G. s. speciosa, are recognized (Dickerman, 1970).

Up to now, five species of Isospora has been identified in parulids of the New World: I. cardellinae in the red warbler (Cardellina rubra) (Salgado-Miranda et al., 2016) and I. celata in the orange-crowned warbler (Leiothlypis celata) (Berto et al., 2014b) in Mexico, I. piacobrai in the masked yellowthroat (Geothlypis aequinoctialis) (Berto et al., 2010) and Isospora basileuterusi in the Basileuterus culicivorus (Mello et al., 2022) in Brazil and I. orbisreinitas in the rufous-capped warbler (Basileuterus rufifrons) in Costa Rica (Keeler et al., 2014). An undescribed isosporoid coccidia has been reported in the common yellowthroat Geothlypis trichas (Boughton et al., 1938) and in the Nashville warbler Leiothlypis ruficapilla (Swayne et al., 1991). In Mexico, C. rubra, L. celata, B. rufifrons and G. trichas, overlap in their distribution with G. speciosa subsp. speciosa. Only L. celata and G. trichas have been recorded in the marshes Ciénegas del Lerma (Soriano-Vargas, 2016).

Materials and methods

During daily trips to marshes Ciénegas del Lerma (19°20′24″N, 99°29′26″W; 19°21′28″N, 99°31′10″W), Mexico, black-polled yellowthroats (G. speciosa) were observed and photographed, from April 15, 2020 to May 15, 2021. Birds were observed eating, moving and perching atop California bulrush (Schoenoplectus californicus) and reed (Thypa sp.). During photographic sessions at different locations, 5 adult males and 2 females left droppings on the leaves. The fecal samples were collected by using a toothpick, avoiding urate deposits and were placed in plastic vials containing 2.5% (w/v) potassium dichromate solution (K2Cr2O7) at a ratio of 1:4 (v/v). Samples were placed in a thin layer (c.5 mm) of K2Cr2O7 2.5% solution in Petri dishes, incubated at 20–26°C and monitored daily under a light microscope (Duszynski & Wilber, 1997). Oöcysts (n = 30) were microscopically examined using the technique described by Duszynski & Wilber (1997) and Berto et al. (2014a). Morphological observations, photomicrographs and measurements were made using a Nikon Eclipse 80i binocular microscope (Nikon Corporation, Tokyo, Japan) coupled to a digital camera Nikon DS-Fi2 (Nikon Corporation, Tokyo, Japan) and a composite line drawing made. All measurements are in micrometers and are given as the range followed by the mean in parentheses.

Results

Three of the seven fecal samples examined contained oöcysts. Two days after the collection of samples, more than 70% of the oöcysts were sporulated (under the conditions used in this study).


Family Eimeriidae Minchin, 1903

Genus Isospora Schneider, 1881


Isospora speciosae n. sp.


Type-host: Geothlypis speciosa Sclater (Aves: Passeriformes: Parulidae), black-polled yellowthroat.

Type-locality: Chimaliapan marsh (DOF, 2018), Ciénegas del Lerma (19°20′24″N, 99°29′26″W; 19°21′28″N, 99°31′10″W), State of Mexico, Mexico.

Type-material: Oöcysts in dichromate solution, phototypes and line drawings of sporulated oöcysts are deposited and available in the Repository (www.ibirds.org) of the Institute for Biodiversity Research, Development & Sustainability (iBIRDS). Photographs of the type-host specimens (symbiotypes) are deposited in the same collection. Photomicrographs of sporulated oöcysts are deposited and available in the Repository of iBIRDS (www.ibirds.org). The repository number is ESV-29/2022.

Prevalence: Oöcysts of this species were found in 3/7 (42%) of the fresh faecal samples examined.

Site of infection: Unknown. Oöcysts were recovered from faeces.

ZooBank registration: To comply with regulations set out in article 8.5 of the amended 2012 version of the International Code of Zoological Nomenclature (ICZN, 2012), details of the new species have been submitted to ZooBank. The Life Science Identifier (LSID) for Isospora speciosa is urn:lsid:zoobank.org:pub:96F2C64C-35E5-4592-BD64-7B057A94DF08.

Etymology: The specific name is derived from the species name of the type-host.

Description (Figs. 1, 2).

Fig. 1
figure 1

Photomicrographs of sporulated oöcysts and sporocysts of Isospora speciosae n. sp. A, Subspherical oöcyst with clearly visible ovoidal sporocysts with a polar granule (PG) between them. PRB, posterior refractile body of the sporozoite; B, Two sporocysts with clearly half-moon shaped Stieda body (SB); C, One fractured oöcyst with clearly visible double outer wall (DOW); D, One sporocyst released from the oöcyst showing sub-Stieda body (SSB) and sporocyst residuum (SR), consisting of many spherules. Scale-bars: 10 µm.

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Fig. 2
figure 2

Line drawing of a sporulated oöcyst of Isospora speciosae n. sp. from Geothlypis speciosa. Scale-bar: 10 µm.

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Sporulated oöcyst

Oöcysts (n = 30) subspherical to ovoidal, 23–28 × 13−27 (26.6 × 25.4). Wall bi-layered, 1.2−1.4 (1.3), outer layer smooth, 1/3 of total thickness; length/width (L/W) ratio 1.0–1.1 (1.1). Micropyle and oöcyst residuum absent. Polar granule present, 1 or 2 (2.0 × 4.0) (Figs. 1, 2).


Sporocyst and sporozoites

Sporocyst (n = 30) are ovoidal, 17–19 × 9–11 (18.7 × 10.2); length/width (L/W) ratio 1.7–1.8 (1.8). Stieda body present, half-moon-shaped (0.5 thick); sub-Stieda body present, trapezoidal, prominent, 1.7 high × 3.0 wide; para-Stieda body absent. Sporocyst residuum present, consisting of many spherules (0.3–0.6) (Fig. 2C). Sporozoites 4, vermiform, 3.3–3.6 × 15.0–16.0, with posterior refractile body (5.5 in length), anterior refractile body (2.8 in diameter) and indiscernible nucleus. Discrete striations (3-5) are present between the posterior refractile body and anterior refractile body (Figs. 1, 2).

Remarks

Seven parulid species have been reported as host of Isospora spp.: Basileuterus culicivorus (Deppe) for I. basileuterusi (see Mello et al., 2022), Cardellina rubra (Swainson) for I. cardellinae (see Salgado-Miranda et al., 2016), Leiothlypis celata (Say) for I. celata (see Berto et al., 2014b), Basileuterus rufifrons (Swainson) for I. orbisreinitas (see Keeler et al., 2014), and Geothlypis aequinoctialis (Gmelin) for I. piacobrai (see Berto et al., 2010). The common yellowthroat Geothlypis trichas (Linnaeus) (see Boughton et al., 1938) and the Nashville warbler Leiothlypis ruficapilla (Wilson) (see Swayne et al., 1991), host for an undescribed isosporoid coccidia. The morphology and morphometry of the oöcysts of I. speciosae allow differentinating it from the other Isospora species reported (Table 1). The mean dimensions of the sporulated oöcysts (25.7 × 22.2) in I. speciosae n. sp. appear to be considerably smaller than those in I. celata (28.4 × 26.4). In I. speciosae the Stieda body is prominent and half-moon-shaped while knob-like in I. basileuterusi, I. cardellinae, I. celata, I. orbisreinitas and I. piacobrai. A polar granule is absent in I. cardellinae and I. celata (Table 1). Striations (3-5) are only present in I. speciosae (Figs. 1, 2; Table 1).

Table 1 Comparative morphology of Isospora spp. recorded from warblers (Parulidae) from the Americas
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Discussion

To date, no helminth or protist parasites have been described in G. speciosa. Of the 115 warbler species that occur in the New World, only seven have been reported as hosts of Isospora spp. as mentioned above. This low frequency may not reflect the distribution and prevalence of Isospora in the New World warblers but is rather an outcome of a small number of studies on the genus Isospora from Parulidae (Berto & Lopes, 2013).

The sporulated oöcysts obtained in this study were compared in detail with coccidian parasites from other New World passerine birds that are feature-similar and belong to the same host family (Table 1) (Duszynski & Wilber, 1997; Berto et al., 2014b). In conclusion, I. speciosae is considered as a species new to science, being the sixth species of the genus described from a New World parulid species.