Morphological and molecular variability of Peridinium volzii Lemmerm. (Peridiniaceae, Dinophyceae) and its relevance for infraspecific taxonomy Author Holzer, Victoria J. C. Department Biologie: Systematics, Biodiversity & Evolution Author Kretschmann, Juliane Department Biologie: Systematics, Biodiversity & Evolution Author Knechtel, Johanna Department Biologie: Systematics, Biodiversity & Evolution Author Owsianny, Paweł M. Nadnotecki Institute UAM in Piła, Adam Mickiewicz Author Gottschling, Marc Department Biologie: Systematics, Biodiversity & Evolution text Organisms Diversity & Evolution 2022 2021-10-09 22 1 1 15 http://dx.doi.org/10.1007/s13127-021-00514-y journal article 10.1007/s13127-021-00514-y 1618-1077 12764891 Peridinium volzii is morphologically variable Despite the uniqueness of P. volzii due to the small first apical plate, the morphological plasticity within the evolutionary lineage appears great. However, all this plasticity (shown within monoclonal strains and partly formalised in numerous varieties and forms) cannot be associated with other traits such as environmental conditions, geographic occurrence or genetic constitution that evolutionary adaptation appears of minor importance in P. volzii . A considerable number of infraspecific taxa have been described ( Table 2 ) and based on the microscopic study of monoclonal strains we confirm the existence of traits such as additional sutures (associated with, e.g., P. volzii forma complexum ; Fig. 3m ) and fusion of plates (associated with, e.g., P. volzii var. simplex ; Fig. 3l ). Other traits such as the shifted suture (of Peridinium guestrowiense var. betacollineatum Er.Lindem. ) having importance in species such as P. cinctum ( Izquierdo López et al., 2018 ; Romeikat et al., 2019 ) have not been found during the course of the present study. It is known today that varying widths of sutures, where the intercalary space becomes distinctively striate, are ontogenetically disposed (growth bands: Netzel, 1982 ). This trait ( Figs. 1a , 3b, e, i, k ) is associated with P. guestrowiense forma late-intercalatum and P. guestrowiense forma latissime-intercalatum Er.Lindem. and is the only one that occurs in combination with other morphological deviations. ) continued ( The diagnostic trait of P. volzii var. australe G.S.West is the antapical plates of unequal size (versus equal size in P. volzii var. volzii ), and this character state is frequent (> 70%) in the material under investigation ( Fig. 3i ). Similarly, the diagnostic trait of P. volzii var. cinctiforme M.Lefèvre is the apical plates 2′ and 4′ of unequal size (versus rather equal size in P. volzii var. volzii ), and this character state (distinctive in species such as P. cinctum and Peridinium raciborskii Wołosz. ) is also not rare (25 ‒ 50%) in the material studied here ( Fig. 3k ). We even confirm the existence of Peridinium guestrowiense subvar. originale Er.Lindem. having six postcingular plates (versus five such plates usually present in peridinialean dinophytes; Fig. 3i ). The reduction from six to five postcingular plates in Peridiniales may result from a fusion of the third and fourth postcingular plate ( Gottschling et al., 2021 ) present today in dinophytes such as the Amphidomataceae ( Tillmann et al., 2009 ). Already Lindemann (1920: 122) pointed out the possibility that phylogenetically ancestral character states occasionally recur (known as atavism), which is corroborated by our observations of individual such cells in monoclonal strains. However, all such variation occurs in individual cells and cannot be associated with other traits such as a genetic constitution or geographic occurrence. Therefore, the recognition of corresponding taxa at infraspecific ranks does not appear justified. 2 Table Table 3 Newly identified phenotypical variabilities in strains determined as P. volzii . Frequency determined for each ribotype ( n = 100)

Variation	Ribotype	Image
Plate 4′′ pentagonal	I (21%), II (3%), III (1%)	n =100 for each ribotype 3b
Plate 2a split	I (3%), II (2%), III (3%)	n =100 for each ribotype 3c
Plates 1a and 3′ fused	I ( 7%), II (6%), III (2%)	n =100 for each ribotype 3f
Plates 2′′ and 3″ fused	I (2%), II (1%), III (1%)	n =100 for each ribotype 3d
Plates 1′′′ and 1′′′′ fused	I (1%), II (1%), III (1%)	n =100 for each ribotype 3e

In addition to the historically described intraspecific variabilities, we detected five cases of yet undiscovered deviation in P. volzii . The unusual epithecal configuration with plate 4″ abutting plate 3a ( Fig. 3 a-b) has to the best of knowledge never been noted in the literature before. More-over, alternate configurations between, for example, plates 3′ and 4″ (resp. 1a and 3a; Fig. 1c, f ) within monoclonal strains are reminiscent of, for example, Parvodinium mixtum Wołosz. ex Kretschmann, Zerdoner, Owsianny & Gottschling ( Kretschmann et al., 2018a ). An additional suture and the split of plate 2a ( Fig. 3c ) have also not been reported from P. volzii so far. Fusion of plates regularly though rarely appears in the thecal pattern of various dinophytes, and corresponding observations in the present study do not come as a surprise. Usually, the fusion of plates takes place within plate series such as the intercalary ( Fig. 3f ), precingular ( Fig. 3d ) and antapical series (associated with P. volzii var. simplex ; Fig. 3l ). However, the fusion of plates associated with different plate series is very rare, and we found fused plates 1′″ and 1″″ ( Fig. 3e ) once only. Anyhow, also the newly discovered variability occurs in individual cells associated with all three ribotypes present in this study and does not make the recognition of corresponding taxa at an infraspecific rank seem justified either.