Vaucheria incurva T.A. Christensen 1986

Vishnyakov, Vasily S., 2023, A new record of Vaucheria incurva (Vaucheriaceae, Xanthophyceae), an extremely rare species, with taxonomic remarks on other Vaucheriae having disintegrated gametangial walls, Phytotaxa 598 (3), pp. 264-268 : 264-267

Vaucheria incurva T.A. Christensen 1986 , Arch. Protistenkd., 132(4): 285.

New locality: — RUSSIA, Krasnodar Territory, the city of Sochi, Adler , 43.448858°N, 39.942791°E, flowerbed in the garden, on bare soil, 8 May 2022, Vishnyakov V-932 .

Morphology: —Filaments are monoecious, attached by sparingly branched rhizoids to the soil substrate, 30–45(53) µm in diameter. Gametangia are born on fruiting branches (gametophores), which appear laterally and terminally and do not any differ in diameter from filaments. Fruiting branches usually consist of 1(2) oogonia borne laterally to 1 antheridium; gametangial pedicels are strongly curved and positioned at ca. 45–90-degree angles apart to each other ( Figs. 1, 2, 4 ). Proliferations of fruiting branches take place commonly from the distal part of oogonial pedicel and lead them to a heap arrangement ( Figs. 1–3 ). Unemptied antheridia were rarely observed, and in such cases, are not clearly separated from pedicels by transverse walls ( Fig. 4 ), and are circinate-cylindrical. The antheridial wall commonly disintegrates soon after discharging, starting at the apex; as a consequence, only more or less preserved remnants can occur at the tip of antheridial pedicel ( Fig. 5 ). The best-preserved empty antheridia are 57.5–70 µm long and 25 µm in diameter. Unemptied oogonia segregating by transverse walls and containing oospores are not observed, however juvenile oogonia obviously lack a distal prominence with a distinctive fertilization pore ( Fig. 4 ). Remnants of the oogonial wall can be rarely observed when fixing oospore at the oogonial pedicel ( Figs. 6, 7 ). Oospores are globose, 32.5–76.25 µm in diameter ( Fig. 8 ), sometimes irregular, and in such cases, have a more angular, plane-convex or concave-convex profile ( Fig. 9 ); one large irregular oospore was 82.5 × 87.5 µm. Oospores are light-brown with scattered drops of yellow-brown pigment. Oospore wall is colorless, rugose, sometimes unevenly thickened, 5–10 µm thick ( Figs. 8–10 ).

The morphology of V. incurva from the new locality generally corresponds to the original description, although it is characterized by somewhat broader filaments, longer antheridia, and a wider range in dimensions of oospores and oospore wall thickness.

Distribution: —The known distribution of V. incurva is limited to regions with subtropical climates. The species was originally described based on specimens from Delphi, Greece (holotype locality) and Adelaide, southern Australia ( Christensen 1986). The new locality, located at the northern border of the subtropics on the Russian coast of the Black Sea, is characterized by a warm, humid climate with a mean annual temperature of approximately 14.5°С and annual precipitation of about 1.5 m ( Rybak et al. 1994, Rybak & Rybak 2017). This locality is extremely remote from the two previously known localities in Greece (~ 1500 km in a straight line) and Australia (~ 13200 km), and represents the northernmost locality for the species. Similar to the previously known localities, the species was discovered in an artificial terrestrial habitat that is exposed to precipitation, and may also receive additional watering as a flowerbed. The sample was collected in early May, during the spring season, when the weather was still relatively cool with an air temperature range of 13–20°C and frequent rainfall. At that time, several other species of Vaucheria , which are commonly found in terrestrial habitats, were also abundantly present in the same and similar habitats in Sochi.

Taxonomic notes: —As in the original material, it was difficult to detect gametangia separated from the vegetative thallus, especially oogonia, in the studied specimens. Christensen (1986) made some assumptions about this fact, considered the rapid disintegration of the oogonial wall after fertilization as one of the most likely explanations. He also suggested that fertilization may occur soon after the liberation of the egg, as it is suggested in V. hercyniana Rieth ex Vishnyakov (2020: 154) , a species of the section “ Hercynianae ”, nom. inval. 1 ( Rieth 1980: 446). The latter species has club-shaped or saccate antheridia and subspherical oogonia both with almost entirely disintegrated walls. Christensen placed V. incurva into the section Heeringia Blum (1971: 193) , which so far has only included a single species, V. uncinata Kützing (1856: 21) , a type for the section. This species is characterized by short-cylindrical or club-shaped antheridia and subspherical oogonia without a distinctive fertilization pore. Although not as rapidly as in V. incurva , this species also has disintegrated gametangial walls ( Rieth 1963, Christensen 1986). In the past, Entwisle (1988) moved V. uncinata to the section Racemosae ( Walz 1865: 212) Entwisle (1988: 32) , which is comprised of species with gametangial walls that remain intact. Recently, I recognized some ambiguities in the sectional system while having the rare opportunity to observe V. hercyniana , an extremely rare European species, and proposed to reinstate Heeringia as a separate section for at least two species, V. uncinata and V. incurva . It was also assumed that the section can be expanded further to include V. hercyniana after thorough studies ( Vishnyakov 2020). This is why there was a need to reconsider the sectional placement of these similar species. Therefore, below, I present new arguments in favor of retaining the section Heeringia along with its two species.

In the genus Vaucheria , the walls of both antheridia and oogonia typically remain intact after discharge and fertilization, respectively. Disintegration of gametangial walls that ensures the contact of gametes has long been considered an important feature for sectional classification, with the sections Globiferae ( Heidinger 1908: 360) Heering (1921: 86), Heeringia , and “ Hercynianae ” being described on the basis of species showing this feature. In his revision of the Australian species, Entwisle (1988) argued that the disintegration of gametangial walls has no importance at the sectional level and placed V. uncinata to the section Racemosae . As a result, the name Heeringia was included in synonymy of Racemosae . However, “ Hercynianae ” was recognized as a separate section, despite the fact that the name was invalidly published. The other species in Racemosae have gametangia with intact walls, featuring circinate-cylindrical antheridia and oogonia characterized by a distinctive fertilization pore or several pores at a distal prominence ( Blum, 1953, 1971; Entwisle 1988). These characteristics are in sharp contrast to those of Heeringia , while Heeringia and “ Hercynianae ” contain indeed similar species.

A set of common features can be identified in “ Hercynianae ” when compared with Heeringia . 1) Antheridia are somewhat similar in shape, being short-cylindrical, club-shaped, or saccate. 2) Oogonia do not have a distinctive fertilization pore, but are globose to broadly ellipsoid and arise laterally to antheridia. 3) Mature oospores are similar in all three species and apparently inherit the shape of the oogonium. 4) As in V. uncinata and V. incurva , gametangial walls disintegrate simultaneously with or soon after sperm release and fertilization in V. hercyniana . The differences are therefore in the degree and pace of disintegration; however, it is difficult to consider them important enough to distinguish between “ Hercynianae ” and Heeringia .

In V. uncinata , remnants of gametangial walls may fix oospores on fruiting branches rather long time and are usually visible at the tips of antheridial pedicels (as depicted in figures from: Rieth 1963, Christensen 1986: figs. 3, 7–9). Remnants of gametangial walls can also sometimes be observed in V. incurva (as shown in Figs. 5–7 and also in Christensen 1986: figs. 14, 17, 18) and V. hercyniana ( Rieth 1974: figs. 2g, 2h, 2i, Vishnyakov 2020: figs. 7–9). Therefore, it is difficult to support the claim that fertilization strongly occurs outside the oogonium, as oospores with a mucilaginous wall attached to the filament are frequently observed. More plausibly, fertilization takes place during extensive, but still incomplete disintegration of gametangial walls in V. incurva and V. hercyniana .

Although it is debatable whether V. hercyniana belongs to Heeringia , a set of features mentioned above separates V. uncinata and V. incurva from Racemosae . One additional matter warrants particular attention in this context. In those other sections that include species having disintegrated gametangial walls, only the antheridium is subjected to disintegration soon after discharge, such as in V. prolifera Dangeard (1939: 297) , a species of Tubuligerae ( Walz 1865: 30) Heering (1907: 132), and V. dillwynii ( Weber & Mohr 1803: 12) C. Agardh (1811: 21) of Corniculatae ( Walz 1865: 21) Heering (1907: 143). In Heeringia , both the antheridium and oogonium disintegrate. This feature, coupled with the absence of а distinctive fertilization pore, distinguishes Heeringia from all other sections in Vaucheria . Subordination of Heeringia to Racemosae seems unnecessary, given that the sections “ Hercynianae ” and Heeringia contain morphologically similar species, while Racemosae and “ Hercynianae ” are recognized as independent in the Entwisle’s system 3. Therefore, V. incurva and V. uncinata are considered members of Heeringia .

Thorough future studies, including molecular analysis, are needed to clarify the infrageneric taxonomy of Vaucheria .

Figures

FIGURES 1–10. Morphology of Vaucheria incurva from new locality: 1–3—general view of proliferating fruiting branches, note multiple proliferations on Figs. 2, 3; 4—detail of fruiting branch with antheridium and oogonium, which are not separated clearly from gametangial pedicels; antheridium is in focus; 5—antheridial pedicel bearing empty antheridium; 6, 7—oogonial pedicels bearing oospores weakly fixed with remnants of oogonial walls; 8–10—mature oospores liberated from oogonial walls. Symbols: a—antheridium, ap—antheridial pedicel, ea—emptied antheridium, fb—fruiting branch, og—oogonium, op—oogonial pedicel, os—oospore, ow—oogonial wall, p—proliferation. Scale bars: 1–3—100 µm, 4–10—30 µm.