The secret behind male ornaments

Date:
December 11, 2020
Source:
University of Wu"rzburg
Summary:
In many species all over the animal kingdom, males have eye-catching
characteristics. Although often impractical, they are beneficial
in finding a mate. Scientists have now mapped the genetic bases
of such a male ornament in a fish.



FULL STORY ==========================================================================
The tail feathers of the peacock, the enormous horn of male rhinoceros
beetles, the protruding antlers of some deer: In nature, there are
countless examples of features which at first sight may only have
disadvantages for their owners.

After all, it is more difficult to hide from a predator when one is
wearing a colourful plumage, and large antlers do not make escaping
in the forest any easier. As a rule, it is the male that has such characteristics.


==========================================================================
The evolution of male ornaments has therefore been fascinating to
biologists since ever. Already Charles Darwin wondered of how such
exaggerated, energy- consuming and in principle harmful structures
could have been created by natural selection. Using the example of
the swordtail fish (Xiphophorus hellerii), he explained his theory of
sexual selection. Darwin's basic idea: If females prefer to mate with
the carriers of striking ornamental traits, such traits might become established in the course of evolution even though they are likely to
be harmful for their owners.

Scientists from Wu"rzburg, Constance and the USA have now been successful
in finding the genetic bases of this evolutionary model in Xiphophorus,
also well known to aquarists as one of their favourite pets. Among all
eligible genes, the researchers identified some that are responsible for
the development of the corresponding ornamental trait in this species
of fish. Their findings also suggest that in the swordtail a gene that
is actually important for neuronal processes in the brain has taken on
an additional new function during evolution.

The scientists published their findings in the journal Current
Biology. Manfred Schartl, a senior professor at the Department of
Developmental Biochemistry at the University of Wu"rzburg, is the lead
author of the study. The project was co-initiated by the evolutionary
biologist Axel Meyer from Constance University with whom the Wu"rzburg researchers have been investigating this phenomenon for many years. For
more than two decades, the two laboratories at the Universities of
Wu"rzburg and Constance have jointly researched the genetic basis of
the sword. The current study has now brought the scientists a big step
closer to understanding the genetic basis of the extended caudal fin
of swordtails.

"In several species of the genus Xiphophorus, the males carry a so-called 'sword', a striking extension of the lower edge of the tail fin, which is yellow, orange or red in colour and surrounded by a dark black margin," explains Manfred Schartl. The sword develops during puberty and can be
as long as the fish itself in some species. This should actually be a disadvantage, because the conspicuous body ornament attracts predators
on the one hand and on the other hand makes escaping more difficult as it reduces swimming performance. However, the females of Xiphophorus hellerii
and several related species prefer to mate with males that carry a long
sword -- males with shorter swords literally lose out in this competition.

Gradually excluding the suspects The genetic bases of this extension
of the caudal fin in Xiphophorus have previously been unknown. However, knowledge of this phenomenon is necessary to test hypotheses about the
role of sexual selection at the molecular genetic level.



==========================================================================
The scientists took a gradual approach to pinpointing the responsible
genes.

They started by looking for all genes that are specifically active in
the sword developing part of the tail fin, but not in fin regions that do
not form a sword. "This process resulted in a set of 329 differentially expressed genes in all sword transcriptomes," said Schartl, describing
the result. The term transcriptome refers to the entirety of genes
that are transcribed in a cell or tissue at a certain point in time,
i.e. are active.

The consideration that genes responsible for sword formation are only
expressed in males led to a significant reduction in the number of
suspects in the next step. The scientists created transcriptomes of
cells from specific areas of the caudal fins in both male and female
specimens. If the females showed comparable activities to males, it was
clear that these genes are not among the sought- after candidates. After
this process, 255 of the original 329 genes remained.

"Interestingly, this comparison revealed that a spatial pattern of
five transcription factors -- Zic1, Hoxb13a, Six2a, Tbx3a and Pax9 --
is responsible for organising the preconditions in the caudal fin for
the development of a sword, and that this pattern is also present in
females," said Schartl.

Backcrossings provide important information Genetic mapping came next
to further reduce the still high number of 255 candidate genes. For this purpose, swordtail males were crossed with females of a related species
whose males had lost their sword in the course of evolution.

The male descendants from this mating have swords of different lengths
due to the mixing of the parental genomes depending on their random
genetic make-up.

Sequencing those genomes using special high-throughput techniques then
made it possible to correlate certain chromosome segments with sword
formation, and those with the list of candidate genes. Finally, three
genes were left over.



========================================================================== Spotlight on one main suspect The gene with the scientific name kcnh8
proved to be crucial for the development of the male characteristic. "This
gene codes for a potassium channel -- a group of channels that play
an important role in particular in the transmission and processing of
stimuli in the nervous system," said Schartl.

The new findings point to a gene with a primary function in neural cells
that was recruited during evolution for developing the male sword about
three to five million years ago, i.e. early during the diversification
of swordtail fishes. The new function is not due to structural changes
within the gene and its product, but to changes in gene regulation.

Indeed, experiments show that kcnh8 in the sword during normal development
and after treatment with male hormones is highly upregulated in the region where the sword is organised. In all other fin areas of the males and
in female caudal fins it is only weakly expressed. In addition, further
studies show a direct correlation between the level of gene expression
of kcnh8 and the length of swords.

Schartl and Meyer received support for their research on potassium
channels from an expected source: botanists from the University of
Wu"rzburg. Rainer Hedrich, who heads the Department of Molecular Plant Physiology and Biophysics, and his colleague, Professor Dietmar Geiger,
have been studying potassium channels for a long time. The techniques
they used -- special patch-clamping methods -- could be easily transferred
from plants to the fish.

Potassium channels transport electrically charged particles and thus cause changes in the membrane potential in cells and tissues. According to the scientists, such channels create tissue-wide bioelectric gradients which
affect the overall structure of the cellular microenvironment. Similar phenomena have been observed in the proliferation of cancer cells and
have led to hypotheses about the importance of ion gradients for growth control. The role of Kcnh8 in the development of the ventral caudal
outgrowth in male swordtails is in good agreement with these models.


========================================================================== Story Source: Materials provided by University_of_Wu"rzburg. Original
written by Gunnar Bartsch. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Manfred Schartl, Susanne Kneitz, Jenny Ormanns, Cornelia Schmidt,
Jennifer L. Anderson, Angel Amores, Julian Catchen, Catherine
Wilson, Dietmar Geiger, Kang Du, Mateo Garcia-Olaza'bal, Sudha
Sudaram, Christoph Winkler, Rainer Hedrich, Wesley C. Warren,
Ronald Walter, Axel Meyer, John H. Postlethwait. The Developmental
and Genetic Architecture of the Sexually Selected Male Ornament
of Swordtails. Current Biology, 2020; DOI: 10.1016/j.cub.2020.11.028 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/12/201211115511.htm

--- up 15 weeks, 4 days, 7 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)