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\begin{document}

\title{Mom and dad are not that different after all: immune modulation as a
prerequisite for the evolution of pregnancy}



\author[1]{Nuno Monteiro}%
\affil[1]{CIBIO}%


\vspace{-1em}



  
  \date{June 16, 2024}


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\selectlanguage{english}
\begin{abstract}
Pregnancy, the post-fertilization period when embryos are incubated
within the body, is a dynamic multistage process that has convergently
evolved in many vertebrates. To increase independence from environmental
fluctuations and protect offspring from predation, challenges had to be
initially overcome. The most obvious, when considering such an intimate
relation between the parent and its semi-allogenic offspring, was the
pressing need to dodge immunity-associated embryo rejection. In mammals,
immunological tolerance was found to be dependent on the active
modulation of the immune system. Even though supporting much of the
current knowledge on vertebrate pregnancy, mammals lack extant
transitional stages that could help reconstruct the evolutionary pathway
of this fascinatingly complex reproduction mode. In this issue of
Molecular Ecology, Parker et al. (2022) selected an untraditional model
- the seahorse and pipefish family, whose species evolved male pregnancy
across an almost continuous gradient of complexity, from external
oviparity to internal gestation. By contrasting gene expression profiles
of syngnathids with distinct brooding architectures, this study allowed
for the observation of subtle evolutionary adaptations, while confirming
the existence of remarkable similarities to `female' pregnancy (e.g.,
the evolution of male pregnancy in pouched species occurred alongside
immune downregulation, and inflammation seems vital during early
pregnancy stages). In a world where the debate on sex-roles takes centre
stage, Parker et al. (2022) appeasing results hint at the fact that the
strongly convergent evolution of vertebrate pregnancy was seemingly
unaffected by which sex carries the burden of gestation.%
\end{abstract}%



\sloppy


\textbf{Mom and dad are not that different after all: immune modulation
as a prerequisite for the evolution of pregnancy}

Nuno Monteiro

CIBIO-InBio, Research Centre in Biodiversity and Genetic Resources,
University of Porto, Vair\selectlanguage{ngerman}ão, Portugal

Department of Biology, Faculty of Sciences, University of Porto, Porto,
Portugal

IUCN SSC Seahorse, Pipefish and Seadragon Specialist Group

nuno.monteiro@cibio.up.pt

\textbf{Pregnancy, the post-fertilization period when embryos are
incubated within the body, is a dynamic multistage process that has
convergently evolved in many vertebrates. To increase independence from
environmental fluctuations and protect offspring from predation,
challenges had to be initially overcome. The most obvious, when
considering such an intimate relation between the parent and its
semi-allogenic offspring, was the pressing need to dodge
immunity-associated embryo rejection. In mammals, immunological
tolerance was found to be dependent on the active modulation of the
immune system. Even though supporting much of the current knowledge on
vertebrate pregnancy, mammals lack extant transitional stages that could
help reconstruct the evolutionary pathway of this fascinatingly complex
reproduction mode. In this issue of Molecular Ecology, Parker et al.
(2022) selected an untraditional model - the seahorse and pipefish
family, whose species evolved male pregnancy across an almost continuous
gradient of complexity, from external oviparity to internal gestation.
By contrasting gene expression profiles of syngnathids with distinct
brooding architectures, this study allowed for the observation of subtle
evolutionary adaptations, while confirming the existence of remarkable
similarities to `female' pregnancy (e.g., the evolution of male
pregnancy in pouched species occurred alongside immune downregulation,
and inflammation seems vital during early pregnancy stages). In a world
where the debate on sex-roles takes centre stage, Parker et al. (2022)
appeasing results hint at the fact that the strongly convergent
evolution of vertebrate pregnancy was seemingly unaffected by which sex
carries the burden of gestation.}

Syngnathids (seahorses, pipefishes, pipehorses and seadragons) belong to
a diverse family of bony fish with more than 300 species, primarily
recognized by its unique mode of reproduction: male pregnancy. This
novelty of the syngnathid lineage long attracted attention from the
scientific community and was rapidly recognised as significant to the

evolutionary radiation of the syngnathid lineage. In fact, one of the
first attempts at reconstructing the phylogenetic relationships within
syngnathids, long before the advent of genomics, was partially grounded
on the general architecture of brooding structures (Herald, 1959).
Curiously, recent efforts, backed by current methodologies (Hamilton et
al., 2017; Stiller et al., 2022), still recognise the diversification of
structures involved in male pregnancy as relevant for diagnosing
syngnathid taxa. From an ancestral pipefish, that probably presented a
rather simple brooding structure where eggs were exposed, increasingly
more complex structures evolved up to the sealed seahorse brood pouch
(Figure 1). This apparent gradient of complexity, easily observable in
extant syngnathid species (and almost absent in mammals), renewed the
interest in this family of quirky fish into as an invaluable opportunity
to shed additional light on the evolution of pregnancy.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1:} Two pregnant syngnathids, a pipefish
(\emph{Syngnathus abaster} ) and a seahorse (\emph{Hippocampus
guttulatus} ; photo by Patrick Louisy).

Even though viviparity has evolved more than 150 times in vertebrates
(Whittington, Griffith, Qi, Thompson, \& Wilson, 2015), most of our
knowledge on the necessary immune system adjustments derives from the
mammals' female pregnancy. Here, immunological tolerance to
semi-allogenic embryos is achieved by the downregulation of both major
histocompatibility complex pathways (MHC I and II). So, what
evolutionary solutions emerged with syngnathid male pregnancy? Recently,
Roth et al. (2020) showed, by reconstructing the immune gene repertoire
of syngnathid genomes, that the evolution of syngnathid pregnancy
coincided with adjustments to the adaptive immune system. In seahorses
(genus \emph{Hippocampus} ) and pipefish (of the genus\emph{Syngnathus}
), both comprising species with a closed brood pouch (where the risk of
embryo rejection would be theoretically higher), several genomic
modifications of the MHC II pathway were uncovered, involving either
loss or modifications of strategic genes. Even though the precise
rearrangement of the MHC II pathway differed between the two types of
studied brood pouches, the end result is similar and indicative of
selection for lower immunological vigilance during the evolution of
pregnancy. Moreover, by analysing the expression of immune and pregnancy
genes in reproductive tissues, Roth et al. (2020) found considerable
overlap between male and female pregnancy, suggestive of a process of
convergent evolution.

The work of Parker et al. (2022), while seemingly flowing from the
leading-edge findings of Small, Harlin-Cognato, and Jones (2013),
Whittington et al. (2015) and Roth et al. (2020), introduced some
relevant methodological upgrades. To fully understand a dynamic process
such as pregnancy, where a myriad of cryptic mechanisms occur in orderly
succession, one needs to go beyond a couple of snapshots. Thus, Parker
et al. (2022) not only extended the inner brood surface tissue gene
expression analyses to four species, thus further solidifying the
validity of their results, but they also contrasted gene expression at
different pregnancy stages (nonpregnant, early, late and parturition),
in three distinct brooding architectures (external egg brooding surface,
inverted brood pouch, and sealed brood pouch; Figure 2).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2:} The three types of brooding structures investigated
by Parker et al. (2022), and the simplified corresponding syngnathid
phylogeny (from left to right: \emph{Nerophis ophidion}
,\emph{Syngnathus typhle} and \emph{Hippocampus guttulatus} . Photos by
Patrick Louisy).

By sequentially monitoring male pregnancy, Parker et al. (2022) were
able to highlight, with great detail, striking similarities between male
and female pregnancy, ranging from: i) the existence of clear
stage-specific gene expression patterns, denoting the necessary
morphological and physiological changes that occur in the pregnant
parent, ii) the need for an inflammatory process during early pregnancy,
likely assisting egg engulfment, or iii) the presence of homologous gene
expression profiles. The apparent absence of male pregnancy specific
genes is particularly interesting, reinforcing Whittington et al. (2015)
observation that pregnancy regulation, despite independently evolving in
clearly divergent evolutionary lineages, seems to use a common `genes
toolkit'.

By contrasting male pregnancy along syngnathids' gradient of brooding
structure complexity, Parker et al. (2022) reinforces the perception
that attaining the highest degrees of intimacy between parent and
offspring (i.e., gestation in a brood pouch) was only possible with
immune modulation. As also observed by Roth et al. (2020), the MHC I
downregulation seems to actively contribute to the necessary
immunological tolerance that circumvents rejection of developing
offspring. While the exact role of MHC II remains ambiguous, we can be
tempted to speculate that its simultaneous loss in pouched syngnathids
is probably much more than a coincidence. Was MHC II dismantlement a
by-product or a prerequisite to the transition from non-pouched to
pouched pregnancy? As pipefishes and seahorses differ in the type of
genomic rearrangements leading to MHC II impairment (Roth et al., 2020),
I presently lean towards the second hypothesis. The future inclusion of
additional syngnathid species, from the pool of intermediate pregnancy
types positioned between external egg gluing and closed pouches, will
surely help shed light on this topic. As an added bonus, a closer look
into the most basal branches of syngnathid phylogeny will also help
pinpoint the unidentified origin, and mode, of nutrient transfer to (and
from) developing embryos. While evidence suggests that pouched species
are able to exchange nutrients to and from their brood (Cunha, Berglund,
Mendes, \& Monteiro, 2018; Sagebakken, Ahnesj\selectlanguage{ngerman}ö, Mobley, Braga Goncalves,
\& Kvarnemo, 2009), the limited data on pouchless species (Miranda,
Vieira, \& Monteiro, 2017; Parker et al., 2022) leads to contradictory
conclusions.

Once viewed as mere oddities, the extraordinary life histories of
syngnathids have long converted them into model organisms to address a
wide array of questions related to the evolution of reproductive
complexity\ldots{} and there are still secrets to uncover. Even if
unable to decisively unveil all the cryptic mechanisms occurring during
male pregnancy, Parker et al. (2022) work significantly progresses our
understanding of the subcellular subtleties resulting from the evolution
of an intimate, and apparently sex-undiscriminating, connection between
offspring and the pregnant mother or father.

REFERENCES

Cunha, M., Berglund, A., Mendes, S., \& Monteiro, N. (2018). The ''Woman
in Red'' Effect: pipefish males curb pregnancies at the sight of an
attractive female. \emph{Proceedings of the Royal Society B, 285}
(1885).

Hamilton, H., Saarman, N., Short, G., Sellas, A. B., Moore, B., Hoang,
T., . . . Brian Simison, W. (2017). Molecular phylogeny and patterns of
diversification in syngnathid fishes. \emph{Mol Phylogenet Evol, 107} ,
388-403. doi:10.1016/j.ympev.2016.10.003

Herald, E. S. (1959). From pipefish to seahorse: a study of phylogenetic
relationships. \emph{Proc Natl Acad Sci U S A, 29} , 465-473.

Miranda, B., Vieira, N., \& Monteiro, N. (2017). Pregnant pipefish with
a simple brooding surface lose less weight when carrying heavier eggs:
evidence of compensation for low oocyte quality? \emph{Acta ethologica,
20} (3), 313-317. doi:10.1007/s10211-017-0268-1

Parker, J., Dubin, A., Schneider, R., Wagner, K. S., Jentoft, S., Bõhne,
A., . . . Roth, O. (2022). Immunological tolerance in the evolution of
male pregnancy. \emph{Mol Ecol, XX} (31), XXX-XXX.

Roth, O., Solbakken, M. H., Torresen, O. K., Bayer, T., Matschiner, M.,
Baalsrud, H. T., . . . Jentoft, S. (2020). Evolution of male pregnancy
associated with remodeling of canonical vertebrate immunity in seahorses
and pipefishes. \emph{Proceedings Of The National Academy Of Sciences Of
The United States Of America, 117} (17), 9431-9439.
doi:10.1073/pnas.1916251117

Sagebakken, G., Ahnesjö, I., Mobley, K. B., Braga Goncalves, I., \&
Kvarnemo, C. (2009). Brooding fathers, not siblings, take up nutrients
from embryos. \emph{Proceedings of the Royal Society B: Biological
Sciences, 277} (1683), 971-977. doi:10.1098/rspb.2009.1767

Small, C. M., Harlin-Cognato, A. D., \& Jones, A. G. (2013). Functional
similarity and molecular divergence of a novel reproductive
transcriptome in two male-pregnant Syngnathus pipefish
species.\emph{Ecol Evol, 3} (12), 4092-4108. doi:10.1002/ece3.763

Stiller, J., Short, G., Hamilton, H., Saarman, N., Longo, S.,
Wainwright, P., . . . Simison, W. B. (2022). Phylogenomic analysis of
Syngnathidae reveals novel relationships, origins of endemic diversity
and variable diversification rates. \emph{BMC Biology, 20} (1), 75.
doi:10.1186/s12915-022-01271-w

Whittington, C. M., Griffith, O. W., Qi, W. H., Thompson, M. B., \&
Wilson, A. B. (2015). Seahorse Brood Pouch Transcriptome Reveals Common
Genes Associated with Vertebrate Pregnancy. \emph{Mol Biol Evol, 32}
(12), 3114-3131. doi:10.1093/molbev/msv177\selectlanguage{english}
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