NSF Tree of Life Program | TOLKIN

LiToL: Assembling the Liverwort Tree of Life

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Life Cycle

Liverworts, like all plants, have a dibiontic life cycle.  A dibiontic life cycle is one that displays two distinct phases that differ in ploidy:  haploid and diploid.  This phenomenon is known as alternation of generations, and the haploid and diploid phases are also called the gametophyte and the sporophyte, respectively.  All land plants have an alternation of multicellular haploid and diploid phases.

Archegonium of Pellia  (photo by K. Renzaglia)

Fossombronia antheridium (photo by K. Renzaglia)

In liverworts, as in other bryophytes (mosses and hornworts), the dominant phase in the life cycle is the haploid gametophyte phase.  That means that when you walk into nature to look at liverworts, you mostly are looking upon haploid tissue.  The gametophyte takes on all of the metabolic functions of the plant, including photosynthesis, gas exchange, and water absorption.  The gametophyte produces specialized structures, gametangia, that produce the gametes (this is where the name “gametophyte” is derived).  Because the gametophyte is haploid, the gametes are made in the gametangia by the process of mitosis and, as a consequence, all the gametes produced by a single gametophyte are genetically identical.  The female gametes (eggs) are produced in gametangia called archegonia, while the male gametes (sperm) are formed in antheridia.

Drawing of liverwort spermatozoid (by K. Renzaglia)

Liverwort sperm cells have multiple flagella that can propel them forward through water to reach the archegonia.  This means, of course, that water is required for successful fertilization.  Once a sperm reaches an archegonium, it swims down the neck of the archegonium to fertilize the egg. 

After successful fertilization, a diploid zygote is formed, which then begins to divide by mitosis to form a multicellular, diploid sporophyte.  The sporophyte is joined to the gametophyte by a foot, through which nutrients are passed from the maternal gametophyte to the young sporophyte.  The sporophyte is completely dependent upon the gametophyte for survival.

Unelongated Riccardia multifida sporophyte (photo by K. Renzaglia)

The sporophyte consists of the foot, a stalk or seta, and a single terminal sporangium. The sporangium consists of two types of cells:  the inner layers of cells are sporogenous tissue (the tissue that will become the spores and elaters), and the outer layer(s) of cells are the jacket (the tissue that surrounds and contains the spores and elaters).  As the sporophyte matures, the cells of the sporogenous tissue divide by meiosis to form many haploid cells.  Some of these haploid cells are spores, and some are elaters.  When this process is complete, the sporophyte is fully mature. The foot, seta, and capsule jacket are diploid, while the spores and elaters inside are haploid.  Keeping track?

Mature sporophyte of Pellia (photo by L. Zhang)

A capsule that has broken open to reveal elaters attached to the capsule valves (photo by J. Shaw)

Spores and elaters of Podomitrium (photo by K. Renzaglia).

The setae of liverworts, in contrast to those of mosses, do not elongate until the sporangium is mature and the spores are ready for release. After full maturity is reached, and when there is plenty of water available, the seta absorbs great quantities of water, which stretches and elongates the component cells.  This stretching and elongation causes the whole sporophyte to become taller and taller, so that the capsule is held high above the mother gametophyte.  The capsule breaks open, and the spores and elaters are exposed.  The elaters stretch and bend, or contract, according humidity levels in the air.  They are very sensitive to slight changes in humditiy and can be observed to move and change shape rapidly. It is thought that perhaps this “wiggling” movement of the elaters aids dispersal of the spores from the sporangium.  The sporophyte doesn’t last long after this; its seta is very fragile, and the structure crumbles and disintegrates.

When the spores land on a suitable substrate in a suitable habitat, they germinate to form a new gametophyte.  The first phase of the gametophyte, called the protonema, is flattened and undifferentiated.  This phase lasts only a short while and is only a few cells large.  Soon, new rhizoids and thallus or leafy stems form.  From here, the whole process begins again!

[INSERT LIFE CYCLE DRAWING FROM RAVEN, AND PHOTO OF GERMINATING SPORES/PROTONEMA?]

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