A prothallus, or prothallium, (from

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

''pro'' = forwards and

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''θαλλος'' (''thallos'') = twig) is usually the

gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...

stage in the life of a

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except ...

or other pteridophyte. Occasionally the term is also used to describe the young gametophyte of a liverwort or peat moss as well. In lichens it refers to the region of the thallus that is free of

alga Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...

e. The prothallus develops from a germinating

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

. It is a short-lived and inconspicuous heart-shaped structure typically 2–5 millimeters wide, with a number of rhizoids (root-like hairs) growing underneath, and the sex organs:

archegonium An archegonium (pl: archegonia), from the ancient Greek ''ἀρχή'' ("beginning") and ''γόνος'' ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gam ...

(female) and

antheridium An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also ...

(male). Appearance varies quite a lot between species. Some are green and conduct

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

while others are colorless and nourish themselves underground as

saprotrophs Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi (f ...

Alternation of generations

Spore-bearing plants, like all plants, go through a life-cycle of alternation of generations. The fully grown sporophyte, what is commonly referred to as the

, produces genetically unique

s in the sori by

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...

. The haploid spores fall from the sporophyte and germinate by

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maint ...

, given the right conditions, into the

stage, the prothallus. The prothallus develops independently for several weeks; it grows sex organs that produce ova (

archegonia An archegonium (pl: archegonia), from the ancient Greek ''ἀρχή'' ("beginning") and ''γόνος'' ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female ga ...

) and flagellated sperm ( antheridia). The sperm are able to swim to the ova for

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...

to form a

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respecti ...

zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...

which divides by mitosis to form a multicellular sporophyte. In the early stages of growth, the sporophyte grows out of the prothallus, depending on it for water supply and nutrition, but develops into a new independent fern, which will produce new spores that will grow into new prothallia etc., thus completing the life cycle of the

organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...

Theoretical advantages of alternation of generations

It has been argued that there is an important evolutionary advantages to the alternation of generations plant life-cycle. By forming a multicellular haploid gametophyte rather than limiting the haploid stage to gametes, there is often only one

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

for any genetic trait. Thus, alleles are not masked by a dominant counterpart (there ''is'' no counterpart). One benefit of this is that a mutation that causes a lethal, or harmful, trait expression will cause the gametophyte to die; thus, the trait cannot be passed on to future generations, preserving the strength of the gene pool. Furthermore, if individual cells of the gametophyte compete with one another, somatic mutations that reduce cell vigour may prevent a cell lineage from reproducing.

In lichens

The region of the thallus in lichens that is free of

e (the

photobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.fungus A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...

(the

mycobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among hypha, filaments of multiple fungus, fungi species in a mutualism (biology), mutualistic relationship.crustose lichens, the prothallus is visible between areoles and on the growing thallus margin. In the large genus '' Cladonia'', the prothallus may provide a mode of

vegetative reproduction Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or spe ...

, and it may have a role in stabilising the soil. In some genera, such as '' Coenogonium'', the presence of absence of prothalli is an important taxonomic character that is used to help classify species. The term prothallus was first used by German botanist Georg Meyer in 1825, who introduced it in a discussion of lichen growth.

References

{{Reflist, refs= {{cite journal , last=Hammer , first=Samuel , title=Prothallus structure in ''Cladonia'' , journal=The Bryologist , volume=99 , issue=2 , year=1996 , doi=10.2307/3244551 , jstor= 3244551 , pages=212–217 {{cite journal , last=Joshi , first=Y , title=A new species and a new record of the lichen genus ''Coenogonium'' (Ostropales: Coenogoniaceae) from South Korea, with a world-wide key to crustose ''Coenogonium'' having prothalli , journal=Mycosphere , volume=6 , issue=6 , year=2015 , doi=10.5943/mycosphere/6/6/3 , pages=667–672 {{cite journal , last=Mitchell , first=M.E. , year=2014 , title=De Bary's legacy: the emergence of differing perspectives on lichen symbiosis , journal=Huntia , volume=15 , issue=1 , pages=5–22 3, url=https://www.huntbotanical.org/admin/uploads/02hibd-huntia-15-1-pp05-22.pdf {{cite book , last1=Ulloa , first1=Miguel , last2=Halin , first2=Richard T. , title=Illustrated Dictionary of Mycology , edition=2nd , year=2012 , publisher=The American Phytopathological Society , location=St. Paul, Minnesota , isbn=978-0-89054-400-6 , page=507

External links

Liverwort SporophyteFern Life-Cycle
Ferns Fungal morphology and anatomy