Mosses in the desert: Fascinating
- What is a moss? (It is not an
alga, not bacteria, not a fungus, not a lichen)
roots or water-storing organs.
plants (one set of chromosomes: xx vs. xy in
eggs and sperm mitotically, not meiotically (as in all seed plants).
sperm dispersed by raindrops.
spores (one-celled) rather than seeds.
- Poikilohydric – can photosynthesize only when wet (in
the presence of water).
are 15,000 species of mosses worldwide – more than either ferns or
gymnosperms. An estimated 100 species in the Mojave Desert.
temperate and warmer climes, mosses typically grow on north-facing rocks
and soils, and may be difficult to find in the field. Why? Carbon Balance
Hypothesis: mosses need to be hydrated long enough to achieve positive
net carbon gain.
- Importance in the desert
for seed germination
- Mosses exhibit some of the most
amazing survival strategies of any plants in the areas of desiccation
tolerance and sexual reproduction.
can lose all its water and still remain alive. No seed plants can be
thoroughly dried out and still survive.
does a desert moss do that and what are its implications?
we can isolate the genes responsible for Desiccation Toelrance
(DT), it is possible to place them into agricultural crops to decrease
mosses can remain totally dry for at least 10 years, and then spring to
life in a matter of seconds upon the addition of water.
mosses dry out, they (1) enclose their cell contents in a sugar gel, and
(2) synthesize repair proteins as transcripts ready to be translated.
DNA – (transcription) mRNA – (translation) proteins. In most organisms,
transcription and translation occur in succession, but it takes time to
go from DNA to protein. The time may be the difference between living and
dying. In desert mosses, the key to survival is the separation of
transcription and translation. When rewetted, the mosses translate reapoir proteins from the transcripts made earlier,
efficiently repairing damaged membranes and chloroplasts.
100 types of repair proteins are made this way.
repair of damage takes about 24 hours, but net photosynthesis can occur
in 20 minutes.
is energetically costly. The currency is carbon. It is thought that the
reason desert mosses grow so slowly is that they bear the cross of DT for
- Sex Ratios
mosses exhibit the most skewed sex ration of any plant on the planet.
1 female:1 male.
XY – meiosis – 1X, 1X, 1Y, 1Y
25 females:1 male
are a “chaste society” compared with flowering plants. Most males never
sire offspring, and >99% of females are virgins for life. With highly
touted advantages for sexual reproduction (genetic variation, repair of
DNA, creation for a dispersal unit), it is puzzling why it is so rare
among mosses. Is sex, then, overrated?
hypothesis of male rarity: “cost of sex.”
<![if !supportLists]>1. <![endif]>In
dioecious seed plants, male-biased sex ratios are the
<![if !supportLists]>2. <![endif]>Females
bear the higher cost of reproduction: a fruit is far more expensive to produce
than is pollen.
<![if !supportLists]>3. <![endif]>Expending
energy to mature a fruit compromises the health of the female, since plants
cannot forage for resources, and resources are limiting.
<![if !supportLists]>4. <![endif]>In
mosses, however, the tables are reversed.
<![if !supportLists]>5. <![endif]> Males incur a higher cost of reproduction –
sperm is far more expensive to produce than eggs.
<![if !supportLists]>6. <![endif]>Males
do not have an “abort switch” to use when resources are in short supply.
<![if !supportLists]>7. <![endif]>It
is likely that this excessive cost borne by the male compromises his ability to
<![if !supportLists]>8. <![endif]>Remember
that >100 repair proteins are synthesized to repair damage from being dry.
These proteins require carbon, and if carbon is needed for growth and
reproduction, less carbon is available to males than to females.