OEL Newsletter Article
(Number 4)
This
is an article that is in the current OEL Newsletter 73 – April 2008.
Those
who do not have the paper copy may like to read it.
Mixed-Up Sexuality and Gender Benders
Dr.
Barbara Corker
Every
autumn, I look to see whether it has been a good year for Ash keys. These brown, winged
seeds of the Ash tree hang in untidy clumps at
the ends of many of the branches in a good year. Most
of them stay there throughout the winter, only gradually being torn off to spin away in
the winter gales. Last year did not seem to
have been a good year for Ash keys (at least around my area) and most of the trees either
lost them or did not produce many in the first place. It must have been a hard winter for
the Bullfinches, for which the Ash keys are a valuable winter food source.
Now
the Ash tree (Fraxinus excelsior) has a
very mixed up sexuality. Most flowering
plants have hermaphrodite flowers, combining both male and female elements in each flower
or alternatively, have separate male and female flowers. These either occur together on
the same plant, or on different plants, giving rise to male and female individuals. Ash is peculiar in that it can have
hermaphrodite, as well as male and female flowers. These can be on the same tree, on
different trees or indeed, any combination of the three. In addition, an individual tree
may be, for example, mostly female one year, but then change sex either partially or
completely the next, a kind of polygamy which is exceedingly rare.
It
is this fluidity in individual sexuality that intrigues me the most. What determines sex
in an Ash tree from one year to the next? Studies
have shown that there is a strong genetic basis to Ash sex determination, but the fact
that the sex of an individual tree can change from one year to another means that
environmental factors must also be involved.
Ash
is by no means the only organism to possess variable sexuality within individuals. Anyone shell collecting on the beach in southern
UK may well have come across another one, the Slipper
Limpet, (Crepidula fomicata). This
animal was accidentally introduced into Britain amongst oysters imported from America in
the 19th C. The shell is a flattened oval with a broad shelf across the
opening. The most distinctive feature is that
individuals often form stacks of shells, one fastened tightly onto the back of another.
There may be 10 or more individuals making up a stack. Larger stacks tend to be curved,
hence the specific name 'fornicata' derived from
the Latin fornix which means 'arch'. The large
limpets at the bottom of the stack are female, the smaller ones at the top are male and,
no surprises here, the ones in between are - well, somewhere in between! The animals change from male to female as they
age, although the first Slipper Limpet to settle will rapidly develop into a female. The
controlling factor behind much of this is that females apparently secrete a hormone into
the water which keeps nearby individuals male.
Another
marine example of gender changing individuals, again from our own coastal waters, is that
of the Cuckoo Wrasse (Labrus bimaculatus). This is a colourful fish that
looks as if it would be more at home on a tropical coral reef than in our grey seas. All
Cuckoo Wrasse start life as females and are a beautiful coral pink with black and white
blotches along their rear dorsal surface. Depending on the proportion of sexes in the
population, some females change into males later in life. The males have a high proportion
of intense, electric blue markings, over a ground colouration of gold. The two sexes look
so different that at one point they were thought to be different species.
So
far, the examples we have looked at have been genetically, environmentally and/or socially
determined, the latter presumably through the controlling influence of excreted hormones.
However, there is another way in which individual organisms may change their sex. This is
through the influence of another organism called Wolbachia, which is a veritable 'gender-bender'.
Wolbachia
is
a
genus of parasitic bacteria which affect the reproduction of an incredible array of
animals, ranging from insects to spiders, mites and nematodes. The bacteria are only
passed on through the eggs of female host individuals.
Depending
on the species of animal which the bacterium infects it can have the following effects:
- It
may kill infected males.
- Infected
males may be infertile or develop as equally infertile pseudofemales.
- It
may cause parthenogenesis, where the eggs of infected females develop without any male
input.
- It
can also create reproductive incompatibility, where infected males can only produce
offspring with females infected with the same strain of Wolbachia.
Wolbachia
has
been found in a staggering number of different animal species. More than 16% of all insect
species in the Neotropics (this zone includes South and Central America, the Mexican
lowlands, the Caribbean islands, and southern Florida) carry the bacterium. It has also
been found in a number of parasitic worms (filarial nematodes), some of which cause
serious disease in humans such as River Blindness (Onchocerciasis). Interestingly,
a considerable part of the negative impact of these nematode parasites seems to be a
result of the host's immune system responding to the Wolbachia
bacteria carried by the nematodes, rather than by the actual nematodes.
Wolbachia
undoubtedly
merits an article in its own right, but suffice it to say here that next time you happen
to come across a Common Pill Woodlouse (as one does!) have a better look at it. It may well be a feminised male or 'neofemale',
as a result of harbouring some of those pesky Wolbachia bacteria. Perhaps,
like me, you will also now stop and look a bit more closely at your neighbourhood Ash tree
each Spring/Autumn to see whether this year it has decided to be 'Arthur' or 'Martha'!
Dr.
Barbara Corker is the Education Officer at the Woodland Education Centre, Offwell. She
regularly writes articles for the newsletter, covering many different and interesting
facets of the natural
world.