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\begin{document}

\title{Ongoing speciation within the diatom Fragilariopsis kerguelensis in the
Southern Ocean?}



\author[1]{Jean-David Grattepanche}%
\affil[1]{Temple University}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\selectlanguage{english}
\begin{abstract}
While the identification of microbial eukaryotes using molecular tools
is now widespread, additional information are needed to confirm the
molecular observation and make the difference between species and
population variants, and therefore to better understand the biogeography
of microbial eukaryotes. In this issue of Molecular Ecology, Postel et
al (2020) not only used three molecular approaches to identify subgroups
of Fragilariopsis kerguelensis but also morphology and physiology to
better understand the relationship between the three genotypes. They
revealed that (1) the three genotypes of the diatom F. kerguelensis have
a negligible gene flux; and (2) two of the genotypes are geographically
isolated with different physiology but still able to crossbreed; and (3)
the last one is omnipresent but reproductively isolated.%
\end{abstract}%



\sloppy


\textbf{Ongoing speciation within the diatom \emph{Fragilariopsis
kerguelensis} in the Southern Ocean?}

Jean-David Grattepanche

Department of Biology, Temple University, Philadelphia, USA

Correspondence:

Jean-David Grattepanche, Department of Biology, Temple University,
Philadelphia, USA. Email: JD.Grattepanche@temple.edu

\textbf{While the identification of microbial eukaryotes using molecular
tools is now widespread, additional information are needed to confirm
the molecular observation and make the difference between species and
population variants, and therefore to better understand the biogeography
of microbial eukaryotes. In this issue of Molecular Ecology, Postel et
al (2020) not only used three molecular approaches to identify subgroups
of \emph{Fragilariopsis kerguelensis} but also morphology and physiology
to better understand the relationship between the three genotypes. They
revealed that (1) the three genotypes of the diatom \emph{F.
kerguelensis} have a negligible gene flux; and (2) two of the genotypes
are geographically isolated with different physiology but still able to
crossbreed; and (3) the last one is omnipresent but reproductively
isolated.}

Microbes in the ocean have long been considered cosmopolitans given
their sizes and the ocean currents with the famous ``Everything is
everywhere, but environment selects'' from Beijerinck (1913) and
Baas-Becking (1934). A lot of works using molecular tools have since
show that the biogeography of microbes is much more complex that what we
expected (Grattepanche, Santoferrara, McManus, \& Katz, 2016; e.g.,
Villarino et al., 2018). For example, de Vargas et al (2015) showed a
worldwide distribution of microbial eukaryotes (or protists) related to
the oceanic regions. But now, we can wonder what the distribution of a
species is within one of these regions or at smaller scale and if the
genetic diversity matches the species delineation. Due to the Antarctic
Circumpolar current, the Southern Ocean is relatively isolated from the
rest of the global ocean and therefore a great location to investigate
the biogeography of protists. In their study, Postel et al (2020) not
only consider the genetic diversity to delineate genotypes within the
diatom \emph{F. kerguelensis} , but how this difference impact their
morphology and physiology.

Postel et al (2020) isolated and cultivated 48 single chains of various
strains of the diatom \emph{F. kerguelensis} collected during a three
months cruise in the Southern Ocean (Figure 1). They later used
ribosomal internal transcribed spacer (ITS) sequencing, double digest
restriction site-associated DNA (ddRAD), and genomic sequencing focusing
on mitochondrial and plastid genomes plus the nuclear ribosomal operon
to identify three subgroups of \emph{F. kerguelensis} . Based on their
molecular analyzes, they showed the absence of genetic material exchange
between these three genotypes. They further looked at these three
genotypes using morphometry, grow rate, photosynthesis efficiency
(maximum photosystem II (PSII) quantum yield), and their reproductive
state (mating preferences). They observed differences in shape and size,
and in adaptability to the light level and temperature. They related
these differences to a biogeographical pattern: a southern genotype (SG)
more adapted to cold and high light conditions and a northern genotype
(NG) adapted to `higher' temperature. The third group (`omnipresent'
genotype or OG), which appear in all location, did not show this kind of
preferences (Figure 2).

A particularly intriguing result from Postel et al (2020) article is the
relationship between the three genotypes. First, the omnipresent
genotype is not able to crossbreed with the two other genotypes, while
the two others, who are able to crossbreed, seem to be isolated even
without any geographical barrier (strong environmental gradient present
in the Southern Ocean). In addition, given the geographical pattern, we
can hypothesize that the `omnipresent' genotype gave birth to the two
other genotypes, but the molecular analyses do not show this. Another
potential hypothesis is that SG and NG separate later, related to the
environmental gradient, but again this is not supported by the molecular
data. In fact, the molecular data show that the SG seemed to have
separate first from their common ancestor, suggesting that the diatom
was adapted first to colder environment with high light. Later, an
`oval' genotype separated in OG and NG, with NG more adapted to warmer
environment and OG more adaptable.

This work is a keystone regarding the old ``Paradox of the plankton''
(Hutchinson, 1961), which try to explain the high diversity in
planktonic organisms in an environment relatively well-mixed. Some
elements of response include patchiness at small scale (Bracco,
Provenzale, \& Scheuring, 2000; Rynearson \& Menden-Deuer, 2016)
assuming that many of the structuring factors cannot be measured (Clark
et al., 2007). In Postel et al (2020) paper, the three genotypes (not
yet considered as different species) showed many physiological and
morphological differences, which have biogeographical incidences (e.g.
temperature and light; Figure 2). While the emergence of SG and NG makes
sense in an ecological and evolutionary framework, the presence of OG is
still intriguing. This work suggests that the survey at smaller scale
will complement the global view from now `classic' molecular survey of
microbial eukaryotes (e.g. de Vargas, Audic, Henry, Decelle, Mahe,
Logares, Lara, Tara Oceans, et al., 2015), to have a better
understanding of the diversity of our microbial neighbors.

Another element is added by the work of Postel et al (2020) regarding
the impact of climate change on microbial eukaryotes. Given the classic
belief that microbes are everywhere and can adapt easily, we tend to
ignore the impact of climate change on these communities. By their work,
Postel et al (2020) showed that some species (or genotypes) do not adapt
easily (e.g. O.G. in the various light and temperature conditions) and
so highlighted the need to consider the microbial eukaryotes within the
climate change topic. Indeed, strong climatic variations in polar
environments may have already had an impact on the diversity of
microbial eukaryotes, especially the diversity we do not know yet
because not assessed by `classic' molecular surveys.

Postel et al. (2020) illustrate how a powerful analytical approach
combining molecular, morphological and physiological aspects allowed
unprecedented insights into complex evolutionary patterns, within a
complex ecosystem to resolve critical question about biogeography in the
Ocean.

Baas-Becking, L. G. M. (1934). \emph{Geobiologie of Inleiding tot de
milieukunde.} The Hague.: W.P. van Stockum and Zoon.

Beijerinck, M. W. (1913). \selectlanguage{ngerman}\emph{De infusies en de ontdekking der
backteriën.} (I. J. van de K. A. v Wetenschappen, ed.). Müller,
Amsterdam.

Bracco, A., Provenzale, A., \& Scheuring, I. (2000). Mesoscale vortices
and the paradox of the plankton. \emph{Proceedings of the Royal Society
of London. Series B: Biological Sciences} , \emph{267} (1454),
1795--1800.

Clark, J. S., Dietze, M., Chakraborty, S., Agarwal, P. K., Ibanez, I.,
LaDeau, S., \& Wolosin, M. (2007). Resolving the biodiversity
paradox.\emph{Ecology Letters} , \emph{10} (8), 647--659. doi:
10.1111/j.1461-0248.2007.01041.x

de Vargas, C., Audic, S., Henry, N., Decelle, J., Mahe, F., Logares, R.,
\ldots{} Karsenti, E. (2015). Ocean plankton. Eukaryotic plankton
diversity in the sunlit ocean. \emph{Science} , \emph{348} (6237),
1261605. doi: 10.1126/science.1261605

de Vargas, C., Audic, S., Henry, N., Decelle, J., Mahe, F., Logares, R.,
\ldots{} Tara Oceans, C. (2015). Eukaryotic plankton diversity in the
sunlit ocean. \emph{Science} , \emph{348} (6237). doi:
10.1126/science.1261605

Grattepanche, J.-D., Santoferrara, L. F., McManus, G. B., \& Katz, L. A.
(2016). Unexpected biodiversity of ciliates in marine samples from below
the photic zone. \emph{Molecular Ecology} , \emph{25} (16). doi:
10.1111/mec.13745

Hutchinson, G. E. (1961). The Paradox of the Plankton. \emph{American
Naturalist} , \emph{95} (882), 137--145. doi: Doi 10.1086/282171

Postel, U., Glemser, B., Salazar Alekseyeva, K., Eggers, S. L., Groth,
M., Glöckner, G., \ldots{} Beszteri, B. (2020). Adaptive divergence
across Southern Ocean gradients in the pelagic diatom Fragilariopsis
kerguelensis. \emph{Molecular Ecology} , \emph{n/a} (n/a). doi:
10.1111/mec.15554

Rynearson, T. A., \& Menden-Deuer, S. (2016). Drivers that structure
biodiversity in the plankton. In \emph{Aquatic microbial ecology and
biogeochemistry: A dual perspective} (pp. 13--24). Springer.

Villarino, E., Watson, J. R., Jönsson, B., Gasol, J. M., Salazar, G.,
Acinas, S. G., \ldots{} Chust, G. (2018). Large-scale ocean connectivity
and planktonic body size. \emph{Nature Communications} , \emph{9} (1),
142. doi: 10.1038/s41467-017-02535-8\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1. \emph{Fragilariopsis kerguelensis} : (a) Scanning electron
microscopic image of a chain (image by Friedel Hinz, Alfred Wegener
Institute); (b) Light micrograph of an offspring chain of the northern
genotype (NG), scale bar, 100 \selectlanguage{greek}μ\selectlanguage{english}m (c) Light micrograph of a clonal cell
culture of the southern genotype (SG).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2. Summary of data collected by Postel et al 2020 showing the
morphological, genetics and physiological differences between the three
genotypes of \emph{Fragilariopsis kerguelensis} (see Postel et al 2020
for more details).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fragilariopsis-SEM-plus-live-2/Fragilariopsis-SEM-plus-live-2}

\end{center}
\end{figure}

\textbf{Hosted file}

\verb`Fig2MEC.pptx`  available at \url{https://authorea.com/users/359729/articles/481556-ongoing-speciation-within-the-diatom-fragilariopsis-kerguelensis-in-the-southern-ocean}

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