Williamsella versus

Rioual, Patrick, Flower, Roger J., Chu, Guoqiang, Lu, Yanbin, Zhang, Zhongyan, Zhu, Bingqi & Yang, Xiaoping, 2017, Observations on a fragilarioid diatom found in inter-dune lakes of the Badain Jaran Desert (Inner Mongolia, China), with a discussion on the newly erected genus Williamsella Graeff, Kociolek & Rushforth, Phytotaxa 329 (1), pp. 28-50 : 42

Williamsella versus Ulnaria (and Tibetiella )

According to Graeff et al. (2013), the fact that W. angusta has a rimoportula at only one apex precludes its inclusion in the genus Ulnaria . In our opinion, the number of rimoportula is not the best criterion to decide to which genus, Ulnaria or Fragilaria , a species should be attributed. This is because there are several species of Ulnaria that possess only one rimoportula as shown for Ulnaria colcae Van de Vijver & Cocquyt (2009: 212) , Ulnaria titicacaensis E. Morales, Ector & P.B. Hamilton (2014: 43) and Ulnaria macilenta E. Morales, C.E. Wetzel & S.F. Rivera (2014: 45) (Morales et al. 2014) while on the contrary, some species of Fragilaria possess two rimoportulae, one at each apex. More importantly in our opinion, W. angusta does not belong to the genus Ulnaria because it has open girdle bands while species of Ulnaria are characterized by having only closed girdle bands. This character represents a synapomorphy for the genus Ulnaria as put forward by Williams (1986, 2011). In recent years, the possession of a closed valvocopula has been used by many authors as a justification for placing new species in the genus Ulnaria (Van de Vijver & Coquyt 2009; Kulikovskiy et al. 2016; Liu et al. 2017). Morales et al. (2014) had argued that it is “difficult to accept closed girdle bands as appropriate evidence” because the type species of the genus Ulnaria , Synedra ulna (Nitzsch) Ehrenberg had not been studied in detail under the SEM. However, soon after Morales et al. (2014) published their study, Lange-Bertalot & Ulrich (2014) proposed an epitype for Ulnaria ulna (Nitzsch) Compère (2010: 100) to solve the problem since the original material from Nitzsch may never be found. This epitype of U. ulna has closed girdle bands ( Lange-Bertalot & Ulrich 2014, Pl. 29, Fig. 3 ).

Similarly, the presence of open girdle bands in W. angusta excludes it from belonging to the monospecific genus Tibetiella , that closely resembles Ulnaria except for the presence of 2–5 rimoportulae on each apex ( Li et al. 2010).

Figures

FIGURES 1–35. Light micrographs for the comparison of Fragilaria crenophila var. sinensis var. nov., Williamsella angusta Graeff, Kociolek & S.R. Rushforth and Fragilaria asiatica Hustedt. Figs 1–15. F. crenophila var. sinensis from lakes in the Badain Jaran Desert. Figure 12 is the holotype. Figs 16–25 W.angusta from Blue Lake, Utah, USA from isotype slide COLO 8508. Figs 26–35. F. asiatica from Tibet, from type slide KA–61. Figs 1–5, untreated material from epiphyton samples (Lake Shaobai Jilin, June 2007), bright field. Figs 6–35 cleaned preparations, taken in bright field and differential interference contrast. Scale bars = 10 μm.