Nitella singaporensis R.E. Romanov, R.S.W. Yeo, B.C. Ho, V.Yu. Nikulin & A.A. Gontcharov, 2020

Romanov, Roman E., Nikulin, Vyacheslav Yu., Yeo, Regina S. W., Ho, Boon-Chuan & Gontcharov, Andrey A., 2020, Nitella singaporensis (Charophyceae, Charales): new species and implications for the taxonomy of the genus Nitella, Phytotaxa 438 (2), pp. 80-94 : 81-86

Nitella singaporensis R.E. Romanov, R.S.W. Yeo, B.C. Ho, V.Yu. Nikulin & A.A. Gontcharov sp. nov.

Type: — SINGAPORE, Upper Pierce Reservoir Forest. Freshwater, edge of the Upper Pierce Reservoir, depth of c. 40 cm, 05-IV-2018, Ho, B. C., Yong, K. T., Liew, D. & Aung Thame SING 2018-407 (holo – SING, iso – LE). Paratype: ibid., 04-VI-2018, Yeo, R. S. W. & Ho, B. C. 18-071 ( SING, LE also in spirit). Det. R. Romanov. ( Fig. 1 ).

Description:—The plants monoecious, multiaxial from the base ( Fig. 1, 2 ), transparent, brilliantly green, not encrusted, 6.5–11 cm in height, with mucilage at the apices ( Figs. 5, 8, 12–16 ) and rhizoids originating from lowest stem nodes. The mucilage covers both the branchlets and stem. Obvious heads are not formed although subtle condensation of uppermost whorls can be noted ( Fig. 5 ). The stem is slender, 270–420 μm in diameter. The internodes slightly shorter, longer (up to 1.5 times) or nearly equal to the branchlets in upper parts and 1–1.5(–2)-times longer in lower ones ( Figs. 1, 2 , 5–7 ).

The notable cells are visible at the lowest stem nodes. They are formed in double number in comparison with branchlet ones ( Figs. 28–30 ). They are merely result of the protrusion of nodal cells at the base of a branchlet in all directions because of node enlargement which is culminating in nodal bulbil formation at the lowest part of the thallus ( Fig. 30 ). The bulbils are up to 1.04 × 0.74 mm.

The sterile and fertile branchlets are similar in principal arrangement, but the former usually are longer and have more lax appearance in contrast with the more or less and finally clearly abbreviated secondary and tertiary rays of fertile branchlets. This results in the compact habit of terminal branchlet parts. The sterile branchlets are situated in the lower part of a plant. The branchlets are arranged in uniform whorls of 6 or 8, rarely 7, 3.4–13.2 mm long, macrodactylous, 2–3-times furcate but never 1- or 4-furcated ( Figs. 3 , 7–22 ). The primary ray is 1.92–6.61 mm long, 86–172 μm across. The longest is ca. ½ of branchlet length or usually slightly longer at lower parts but increasing in relative length towards apex from ca. ½ to 3/5 and 2/3, i.e. 0.37–0.74 of total branchlet length.

The lateral secondary rays are mainly 5 and 6, but 4, 7 and even 8 occur exceptionally. A central secondary ray is also present ( Figs. 7–22 ). The lateral secondary segments are commonly once-forked ( Figs. 9, 14, 16 , 21 ), but may also be 1–2-times forked ( Fig. 12, 15 , 22 ). One or even 3 lateral secondary rays can be unforked as exceptions ( Fig. 17 , arrowheads). In case of single furcation, (4–)5(–6) equal dactyls (in other words, end lateral tertiary rays) surrounding 0 or 1 central dactyl are produced. In case of 2-times furcation, which usually does not form in all lateral secondary segments on the same branchlet (0–4 among 4–7 ones), two variants of third (distal) furcation are possible within the same branchlet. In the first variant, which is more common, there is absence of central dactyl and formation of 4 once-furcated lateral tertiary segments producing (3–)4–5 equal dactyls at the furcation ( Fig. 13 ). In the second, which is rarer, there is a formation of central dactyl (in other words, end unforked central tertiary ray) surrounded with 5 lateral dactyls (i.e. end lateral quaternary rays). The lateral tertiary rays are shorter than dactyls, i.e. less than 1/3 of their length, ca. 0.8 to 2-times shorter. The central dactyl is formed at sterile nodes only, i.e. it is produced instead of antheridium. Therefore, the dactyls of secondary lateral segments range from 4–6 in number, one of them can be central one.

The central secondary ray may be short, resulting in a small length of this part of a branchlet shorter than surrounding lateral secondary segments and nearly hidden with them ( Figs. 4 , 18–21 ), especially if they are adpressed ( Fig. 19 ). As such, the branchlet is tassel-like in general appearance ( Fig. 19 ). In one instance, abbreviated central secondary ray resulted in the appearance of the whole central secondary segment as an accessory branchlet ( Figs. 20, 21 ). Otherwise, they are usually longer where the central part of a branchlet is more or less extending from surrounding lateral secondary segments and looking as a branch ( Figs. 10–15 ). Therefore, in these cases, the branchlets can be considered monopodial. Sometimes the central tertiary rays can be formed at upper furcation of central secondary rays ( Figs. 10, 11 ). They end with furcation producing 3 or 5 dactyls. The absence of node at the top of central secondary ray can occur as an exception. In this case central, the whole secondary ray becomes unusually long and robust dactyl ( Fig. 9 , arrowheads). The central secondary ray is up to 0.64–0.97 mm in length or significantly shorter than lateral ones. Usually it is somewhat stouter in comparison with lateral ones.

The lateral tertiary rays at the top of central secondary ray are 5 or 6 in number. They can be obviously shortened in the same way as in lateral secondary rays as described above, with 0.5 of dactyl’s length. The lateral tertiary segments at central secondary ray may produce a node furcating in 4 or 5 dactyls, or can be nodeless (i.e. they are 5 or 6 dactyls themselves) in the same branchlet. The 1–5 of 5 lateral tertiary rays can ends with furcation. In this case the whole tertiary segment is nearly equal or somewhat longer than dactyls from the same node. In the last case dactyls of the third furcation are obviously shorter than dactyls of second furcation.

The lateral secondary rays are 0.4–2.54 mm in length; the central tertiary ones (if present) are 1.02–3.63 mm; the lateral tertiary ones (if present) are 0.27–1.71 mm; the dactyls are (0.39–)0.64–1.3(–1.65) mm in length.

Dactyls are constantly two-celled ( Figs. 23–25 ), mostly clearly uniform, equal to each other, never abbreviated, with small conical end cell, which seems to be decaying after short-term storage in the laboratory. The first furcation of a branchlet usually does not form dactyls , they are present at second and third furcations generally. The penultimate cell of a dactyl is cylindrical shortly pointed at the apex; therefore, its apex does not differ with the apex of end cell with width ( Figs. 23, 25 ), (37–)57–96(–104) μm width. Terminal (i.e. end) cell is confluent, but is small in comparison with the adjacent penultimate cell, 71–91 × 20–27 μm (rewetted), its apex pointed and mostly clearly thickened. Its thickness varies within (4.3–)6–23 μm.

The gametangia are never formed at the base of the whorls and first furcation (because of constant formation of central secondary ray), or at furcation with central ray. In other words, the nodes with gametangia do not form central ray. Gametangia conjoined, but the samples studied have prevalence of dichogamous (?) nodes. Nodes with single oogonium and even single antheridium are found but it is uncertain without studying its different developmental stages if they were produced without their counterparts, or always together but the antheridia have already fallen at the time of sampling. Both antheridia and oogonia are solitary, not geminate, mainly at the terminal furcations ( Fig. 4 , 26 ). The formation of antheridium on penultimate furcations has been found as a rare case too ( Fig. 26 ). The oogonia can be produced at the second and mostly at the third and the fourth furcations of the branchlet, i.e. from the node at the top of lateral secondary ray in case of absence of central tertiary ray, or at the end furcations of central and lateral secondary segments. The antheridia are found on the youngest branchlets, they are shortly stipitate and nearly equal in appearance to conjoining oogonia ( Fig. 26 ).

Oogonium without coronula 393–429 × 329–340 μm, with 7–8 convolutions ( Fig. 26 ), antheridia 229–234 mm in diameter. Ripe oospores are abundant, dark-brown in fixed state, but yellow-brown in transmission light and black in dry state in reflected light, 275–300 × 239–243 μm. The oospores have 8 low spiral ridges ( Figs. 32, 33 ). The oospore surface sculpture is complex, appearing densely but imperfectly reticulated in light microscope ( LM) ( Fig. 27 ), while in the SEM the reticulation is ornamented with numerous and irregularly distributed granular protrusions of different sizes ( Figs. 33–37 ). The ribs are ornamented with the same type of protrusions ( Figs. 33, 34 ).

Figures

FIGURE 1–4. General appearance and whorl and branchlet of Nitella singaporensis: 1—pressed and dried material (holotype: SING); 2—pressed and dried material (isotype: LE); 3—reconstruction of general habit of branchlet whorl (view from above) from the photo of ethanol-fixed specimen (isotype: LE) showing mean of terms used for branchlet description: 1—primary ray, 2—lateral secondary ray, 3—central secondary ray, 4—lateral tertiary ray, 5—central tertiary ray, 6—dactyls, yellow—lateral secondary segments, green—central secondary segment, blue—central tertiary segment, orange—lateral tertiary segment; 4—fertile branchlet of rewetted herbarium specimen showing oogonia arrangement (isotype: LE). Scale: 1 mm. Scale: 2—1 cm, 4—1 mm. Photo 1 by D. Liew, drawing and other photos by R. Romanov.

FIGURES 5–19. The whorls and branchlets of Nitella singaporensis (all from isotype: LE): 5—apical part with mucilage recognised because of adhering of fine particles at its surface; 5–7—whorls of branchlets, showing long primary ray; 8–18—branchlets, showing variability in length of central secondary ray, from moderately elongated, i.e. exceeding the length of lateral secondary rays with their dactyls (8 & 9), nearly equal with them (10, 12, 14), to abbreviated, i.e. clearly shorter than lateral second segments (19), mainly spreading (8–16) or adpressed to the central one, resulting in last case in tassel-like general appearance of the branchlet (19); 8, 12, 15, 16—fine particles adhered to the surface of a branchlet, indicating presence of a mucilage at its surface; actually they are outlining the mucilage patches; 9—central secondary ray without furcation, appearing as long robust dactyl (arrowheads); 10 & 11—branchlets with central tertiary ray (arrowheads); 11—branchlet with central secondary ray with furcation (double arrowhead), producing central tertiary ray with 3 dactyls (arrowhead); 12—dactyls at the furcation at the top of central secondary ray (arrowhead); 13—central tertiary ray (arrowhead) at lateral secondary ray; 17—unfurcated lateral secondary rays (arrowheads), central secondary ray at the branchlet below is not clearly visible because of artificial deformation at its base. Scale: 5–11—2 mm, 12–17—1 mm, 18 & 19—0.5 mm.All photos by R. Romanov.

FIGURES 20–31. Nitella singaporensis from Singapore (isotype: LE), N. gracilis from Sicily (LE): 20 & 21—abbreviated central second segment (arrowhead), looking as an additional branchlet at first furcation; 21—rare case of 8 lateral secondary rays; 22—end parts of a branchlet with clearly visible central secondary ray (arrowhead); 23–25—2-celled dactyls, some of them appearing 1-celled after loss of end cell during storage in living state in the laboratory; 26—gametangia, [arrowhead—shortly stipitate antheridia, double arrowhead— production of antheridium in penultimate not end furcation], 27—oospore surface in LM, appearing as unclear reticulations, 28–30—stem nodes with easily visible nodal cell extruding from the base of branchlets below the whorl, resulting in nodal bulbil formation (30), 31—stem node of N. gracilis with nodal cell extruding from the base of branchlets and situated around it. Scale: 20–22—0.5 mm, 23–26, 28–31—0.2 mm, 27—20 μm. All photos by R. Romanov, except 26 by R.S.W. Yeo.

FIGURES 32–37. Oospores of Nitella singaporensis (isotype: LE): 32—general habit, 33–37—surface ornamentation of fossa and ribs at different magnification. Scale: 32—40 μm, 33, 34—10 μm, 35—8 μm, 36—4 μm, 37—1 μm. All photos by R. Romanov.