A prothallus, or prothallium, (from

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

''pro'' = forwards and

Greek Greek may refer to: 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 of all kno ...

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

gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the se ...

stage in the life of a

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

or other

pteridophyte A pteridophyte is a vascular plant (with xylem and phloem) that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as " cryptogams", meaning that their means of reproduction is ...

. Occasionally the term is also used to describe the young gametophyte of a

liverwort Liverworts are a group of non-vascular land plants forming the division Marchantiophyta (). They may also be referred to as hepatics. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry ...

or peat moss as well. In

lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...

s it refers to the region of the thallus that is free of algae. The prothallus develops from a germinating

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

. It is a short-lived and inconspicuous heart-shaped structure typically 2–5 millimeters wide, with a number of

rhizoid Rhizoids are protuberances that extend from the lower epidermal cells of bryophytes and algae. They are similar in structure and function to the root hairs of vascular land plants. Similar structures are formed by some fungi. Rhizoids may be un ...

s (root-like hairs) growing underneath, and the sex organs: archegonium (female) and antheridium (male). Appearance varies quite a lot between species. Some are green and conduct

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

while others are colorless and nourish themselves underground as saprotrophs.

Alternation of generations

Spore-bearing plants, like all plants, go through a life-cycle of

alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of life cycle in plants and algae. In plants both phases are multicellular: the haploid sexual phase – the gametophyte – alternates with a diploi ...

. The fully grown

sporophyte A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...

, what is commonly referred to as the

, produces genetically unique

s in the sori by

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

. The

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

spores fall from the sporophyte and germinate by

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

, given the right conditions, into the

stage, the prothallus. The prothallus develops independently for several weeks; it grows sex organs that produce ova ( archegonia) and flagellated sperm ( antheridia). The sperm are able to swim to the ova for

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

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. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

zygote A zygote (; , ) is a eukaryote, eukaryotic cell (biology), 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 individ ...

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 An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

Theoretical advantages of alternation of generations

It has been argued that there is an important evolutionary advantages to the

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 is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...

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 mutation A somatic mutation is a change in the DNA sequence of a somatic cell of a multicellular organism with dedicated reproductive cells; that is, any mutation that occurs in a cell other than a gamete, germ cell, or gametocyte. Unlike germline muta ...

s that reduce cell vigour may prevent a cell lineage from reproducing.

In lichens

The region of the thallus in lichens that is free of algae (the photobiont partner) and contains only

fungus A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

(the mycobiont partner) is called the prothallus. It is typically white, brown, or black in colour. In

crustose lichen Crustose lichens are lichens that form a crust which strongly adheres to the Substrate (biology), substrate (soil, rock, tree bark, etc.), making separation from the substrate impossible without destruction. The basic structure of crustose lichen ...

s, 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 a form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or specializ ...

, 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, doi-access=free {{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