Bryophytes (Mosses)


Mosses in the desert: Fascinating features


  1. What is a moss? (It is not an alga, not bacteria, not a fungus, not a lichen)
    1. No roots or water-storing organs.
    2. No true xylem.
    3. Haploid plants (one set of chromosomes: xx vs. xy in diploids).
    4. Produces eggs and sperm mitotically, not meiotically (as in all seed plants).
    5. Free-swimming sperm dispersed by raindrops.
    6. Produces spores (one-celled) rather than seeds.
    7. Poikilohydric – can photosynthesize only when wet (in the presence of water).
    8. There are 15,000 species of mosses worldwide – more than either ferns or gymnosperms. An estimated 100 species in the Mojave Desert.
    9. In 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.
  2. Importance in the desert
    1. Nutrient cycling
    2. Soil erosion
    3. Water retention
    4. Habitat for seed germination
  3. Mosses exhibit some of the most amazing survival strategies of any plants in the areas of desiccation tolerance and sexual reproduction.
    1. Mosses can lose all its water and still remain alive. No seed plants can be thoroughly dried out and still survive.
    2. How does a desert moss do that and what are its implications?
    3. If we can isolate the genes responsible for Desiccation  Toelrance (DT), it is possible to place them into agricultural crops to decrease irrigation needs.
    4. Desert 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.
    5. As mosses dry out, they (1) enclose their cell contents in a sugar gel, and (2) synthesize repair proteins as transcripts ready to be translated.
    6. Review: 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.
    7. Nearly 100 types of repair proteins are made this way.
    8. Complete repair of damage takes about 24 hours, but net photosynthesis can occur in 20 minutes.
    9. DT 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 survival.
  4. Sex Ratios
    1. Desert mosses exhibit the most skewed sex ration of any plant on the planet.
    2. Expectation: 1 female:1 male.  XY – meiosis – 1X, 1X, 1Y, 1Y
    3. Reality: 25 females:1 male
    4. Mosses 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?
    5. Primary hypothesis of male rarity: “cost of sex.”

1.      In dioecious seed plants, male-biased sex ratios are the pattern.

2.      Females bear the higher cost of reproduction: a fruit is far more expensive to produce than is pollen.

3.      Expending energy to mature a fruit compromises the health of the female, since plants cannot forage for resources, and resources are limiting.

4.      In mosses, however, the tables are reversed.

5.       Males incur a higher cost of reproduction – sperm is far more expensive to produce than eggs.

6.      Males do not have an “abort switch” to use when resources are in short supply.

7.      It is likely that this excessive cost borne by the male compromises his ability to tolerate desiccation.

8.      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.