Isoetes tamaulipana Mora-Olivo, A. Mend. & Mart.

Mora-Olivo, Arturo, Mendoza-Ruiz, Aniceto & Martínez-Ávalos, José Guadalupe, 2016, Isoetes tamaulipana (Isoetaceae), a new species from Mexico, Phytotaxa 267 (2), pp. 113-120 : 115-119

Isoetes tamaulipana Mora-Olivo, A. Mend. & Mart. -Aval., sp. nov. ( Figs. 1–3 )

Isoetes tamaulipana is most similar to I. mexicana in length, width and color of leaves, but differs in having rootstocks 3 to 4 lobed (vs. 2 lobed in I. mexicana ) and a velum covering 3/4 of the sporangium (vs. covering 1/3 of the sporangium). It further differs from this and other species in having light brown or greyish megaspores with a smooth to rugulate perispore and thin laesural walls, laesurae and proximal face of the equatorial ridge bearing tiny spines, and microspores being grey and echinate (vs. megaspores white to pale grey, smooth to weakly tuberculate, microspores grey and tuberculate-verrucose in I. mexicana and other similar species).

Type:— MEXICO. Tamaulipas: Municipio de San Carlos, aproximadamente 20 km W de San Carlos, ladera W del cerro El Diente, vegetación acuática en depresión de afloramiento rocoso de granito, rodeado por bosque de pino, 24°31’20.64” N, 98°57’48.61” W, 1071 m, 11 July 2012 – 17 March 2014, A. Mora-Olivo 13834 (holotype UAT, isotypes IEB, MEXU, UAMIZ).

Perennials; aquatic plants rooted in substrate with emergent leaves; rootstock semi-globose, 3–4 lobed, 10–25 mm wide, 6–10 mm tall, without scales, leaves 15–30 per plant, erect and compactly arranged, 15–25 cm long, 3.5– 6.0 mm wide at the base, 1.5–2.0 mm wide at the half the length, sub-terete in transversal section, basal part with hyaline alae, tapering to the apex, flexible, green when fresh or yellowish when dried; sporangia with translucent walls, globose or elliptical to obovate in the adaxial face, velum covering 75% of the macrosporangium, and 40% of the microsporangium, ligulae triangular and membranaceous; megasporangia white when fresh or pale brown when dried; microsporangia dark colored when fresh or greyish when dried. Megaspores tetrahedral, trilete, 380–400 μm in diameter, light brown to greyish, proximal and distal face smooth or slightly wavy; length of the lasural arms 0.22–0.26 μm, laesural arms 18–25 μm wide, 15–20 μm tall, with tiny spines besides the laesurae and proximal to the equatorial ridge; equatorial ridge 18–20 μm wide. Microspores bilateral, cristate to echinate, greyish in mass, 27–30 μm long, 17–20 μm wide.

Distribution and ecology:— Isoetes tamaulipana is currently known only from the pools located at “Cerro El Diente” located west of San Carlos, in the northern part of the state of Tamaulipas, Mexico. Plants are emergent in small pools formed in granitic rocks ( Fig. 1A ), with roots 5–15 cm deep in the substrate, and are associated with submerged plants such as Elatine Linnaeus (1753: 367) ( Elatinaceae ).

Etymology:— Epithet refers to the Mexican state where the species was discovered.

Discussion:— Isoetes tamaulipana is the first record for the family Isoetaceae in the state of Tamaulipas and is the only known species from northeastern Mexico ( Mora-Olivo & Villaseñor 2007).

It is important to point out that plants of Isoetes are always associated with moist environments and only a few species grow in seasonal ponds in basaltic or granitic outcrops. The flora that develops in this type of substrate has been widely studied in the United States of America ( Singhurst et al. 2011) and several species of Isoetes are associated with these habitats such as I. melanospora Engelmann (1878: 1) , I. tegetiformans Rury (1978: 108) , and I. piedmontana ( Pfeiffer 1939: 411) Reed (1965: 392) ( Heafner & Bray, 2005, Brunton & Britton 2006, Singhurst et al. 2011). However, I. orcuttii Eaton (1900: 13) from Baja California is the only species known in Mexico previously known from this kind of habitat.

Table 1 provides a comparative analysis of the features of known species of Isoetes that grow in habitats similar to that of I. tamaulipana , or that have been described recently and share some characteristics with I. tamaulipana . However, the new species is easily distinguished by its light brown or greyish megaspores with a smooth or slightly wavy exospore, thin laesure, laesure and proximal face of the cingulum with tiny spines (perispore), and microspores that are grey and echinate.

Because of its restricted distribution and its specialized aquatic habitat, Isoetes tamaulipana might be considered an endangered or vulnerable species. Future plans to implement a Natural Protected Area at the Cerro El Diente in Sierra de San Carlos, Tamaulipas could help to conserve and protect this and other species in the area.

Figures

FIGURE 1. Isoetes tamaulipana. A. Habit. B. Plant. C. Basal region of the plant. co= corm, l= leaves, r= roots.

FIGURE2.SporophyllsofIsoetestamaulipana.A.Sporophylls.B.Sporophyllwithmegasporangium.C.Sporophyllwithmicrosporangium. li= ligulae, me= megasporangium, mi= microsporangium, sp= sporophylls, v= velum.

FIGURE 3. Scanning electron micrographs of Isoetes tamaulipana, A–B. Megaspore proximal and distal view. C. Close up of trilete laesura with tiny spines. D. Close up of cingulum with tiny spines. E. Microspores with proximal and distal views. F. Microspore ornamentation. c= cingulum, l= laesure.