Sporolithon, Heydrich, 1897

Maneveldt, G. W., Gabrielson, P. W. & Kangwe, J., 2017, Sporolithon indopacificum sp. nov. (Sporolithales, Rhodophyta) from tropical western Indian and western Pacific oceans: First report, confirmed by DNA sequence data, of a widely distributed species of Sporolithon, Phytotaxa 326 (2), pp. 115-128 : 123-124

Delineation in Sporolithon

Two genera currently are recognized in Sporolithales, Heydrichia and Sporolithon ( Richards et al. 2017) . The defining suite of features of Sporolithon , all possessed by S. indopacificum , are: 1) cells of contiguous filaments commonly joined by both cell fusions and secondary pit connections; 2) tetra/bisporangia borne on single stalk cells that do not produce successive sporangia; and 3) absence of an involucre of narrow, elongate cells surrounding tetra/ bisporangial chambers that differ from other vegetative cells ( Maneveldt & van der Merwe 2012). Within Sporolithon , S. indopacificum is characterised by the following combination of features: 1) free-living rhodoliths that are lumpy with crowded, mostly contiguous, swollen protuberances; 2) rhodoliths comprising individual, variably thick crusts growing in a superimposed manner; 3) thallus construction monomerous with a consistently thin, plumose medulla; 4) contiguous filaments joined primarily by secondary pit connections, cell fusions rare; 5) tetra/bisporangial chambers clustered in sori that are raised 3–5 cells above the surrounding vegetative surface, but that have chamber floors flush with, to sunken 4 cells below the surrounding vegetative surface; 6) tetra/bisporangial chambers 85–110 μm in height and 30–45 μm in diameter; 7) a layer of elongate cells at the base of tetra/bisporangial chambers; 8) tetra/bisporangial chambers separated by 1–3 (mostly 1) sterile paraphyses that are 3–5 cells in length, and that are no different to vegetative cells immediately surrounding the chambers; 9) tetra/bisporangial chamber pores 8–17 μm in diameter and surrounded by 8–13 rosette cells; 10) tetra/bisporangial chambers not becoming buried in the thallus; and 11) having unique psb A and rbc L sequences.

DNA sequences from the rbc L and psb A genes indicate that S. indopacificum is a distinct species. Sporolithon elevatum M.C.Henriques & Riosmena-Rodriguez , S. episporum , and S. tenue Bahia, Amado-Filho, Maneveldt & W.H.Adey, like S. indopacificum , have their tetra/bisporangial chambers in elevated sori that are sloughed; all other Sporolithon species have buried tetrasporangial chambers ( Table 2). This feature, sloughing of the tetra/bisporangial sori, appears to have arisen at least twice in Sporolithon species ( Figs 1 , 2 ). It may have arisen multiple times, but for many sequenced specimens morpho-anatomical data is lacking and vice versa. Although S. durum was reported to slough its tetrasporangial chambers ( Townsend et al. 1995), this observation was not based on type material, which is male. Moreover, DNA sequencing has demonstrated that multiple species are passing under S. durum ( Figs 1 , 2 ; Richards et al. 2017, Gabrielson pers. obs.), including in South Australia, site of the species’ type locality, Cape Jaffe. Thus, a meaningful anatomical comparison cannot currently be made between S. indopacificum and S. durum . Of the species that slough their tetra/bisporangial sori, S. indopacificum differs anatomically from S. elevatum in having monomerous versus dimerous thallus construction and larger tetra/bisporangial chambers ( Henriques et al. 2014); from S. episporum in having longer tetra/bisporangia, taller tetra/bisporangial chambers, and smaller diameter of tetra/bisporangial chamber pores ( Keats & Chamberlain 1993); and from S. tenue in having a vegetative thallus that is substantially thicker than 250 μm and in possessing a layer of elongate cells at the base of the tetra/bisporangial chambers ( Bahia et al. 2014b). Richards et al. (2017) questioned the validity of this last feature for distinguishing S. molle from S. ptychoides and here we suggest a similar caution. Sporolithon indopacificum most closely resembles S. episporum differing in three measured tetra/bisporangial characters ( Table 2); the two species, however, have sequence divergence values that indicate they are distinct species, with 3.1% divergence in psb A and 2.7% in rbc L ( Tables S1, S 2). This likely accounts for the report of S. episporum from Tanzania by Oliveira et al. (2005), and for the Fijian specimen (UWC 94/1265) filed under that name (D.W. Keats, herbarium records) in UWC.

The vast majority of the validly described, extant, Indo-West Pacific species of Sporolithon remain poorly characterized. Many of these are known only from their type collections ( Guiry & Guiry 2017). Since we now know that S. indopacificum has a disjunct distribution in the Indo-West Pacific, we need to obtain species diagnostic DNA sequences from the type specimens of these other named species.

Figures

FIGURE 1. Phylogram of Sporolithon species inferred by maximum likelihood analysis of psbA sequences; Heydrichia species were the outgroup; sequences identified by GenBank accession number. Bolded scientific names are type specimens, topotype specimens, or specimens linked to type specimens by DNA sequence. Bootstrap support values (in %) are provided for nodes where> 50%. Scale bar refers to substitutions per site.

FIGURE 2. Phylogram of Sporolithon species inferred by maximum likelihood analysis of rbcL sequences; Heydrichia species were the outgroup; sequences identified by GenBank accession number. Bolded scientific names are type specimens, topotype specimens, or specimens linked to type specimens by DNA sequence. Bootstrap support values (in %) are provided for nodes where> 50%. Scale bar refers to substitutions per site.