Serotaxonomy: Definition, History and Roles | Plant Taxonomy

In this article we will discuss about Serotaxonomy:- 1. Definition of Serotaxonomy 2. History of Serotaxonomy 3. Methods 4. Roles.

Definition of Serotaxonomy:

Serology is defined as that portion of biology, which is concerned with the nature and interactions of antigenic material and antibodies. Smith (1976) defined it as “the study of the origins and properties of antisera.” When foreign cells or particles (antigens) are introduced into an organism, antibodies are produced in the blood (antiserum).

The substance capable of stimulating the formation of an antibody is called antigen and the highly specific protein molecule produced by plasma cells in the immune system in response to the antigen is called antibody.

Proteins most widely used as antigens in serotaxonomy are those, which carry useful taxonomic information and are easy to handle. Both structural and reserve proteins can be used in the field of systematics, as long as they belong to the same group and the same organs are always compared.

Generally, storage proteins are most amenable for taxonomic studies followed by pollen proteins. Stem tubers, algal cells, fern spores, fruits and leaves can also be employed as satisfactory antigenic material for systematic investigations.

History of Serotaxonomy:

Phytoserology, which deals with immunochemical reactions, between serum antibodies and antigens, has also established itself as a valid method in systematics, because it helps to detect homologous proteins.

It uses the specific properties of antisera produced by animals against plant proteins as characters to assess plant relationships. Serotaxonomy developed and became popular in Germany, which has been an active center since the beginning of this century.

Nuttal was the first biologist to compare the immunochemical specificity of serum proteins for systematic purposes. Kowarski, Bertarelli  and Magnus were the other early notable serologists, who compared proteins from various grass and legume species, showing similarities and differences.

Their experiments gave phytoserology a hopeful start, but unfortunately due to inadequate methods and extraordinary claims of the Konigsberg school, founded in 1914 and headed by Gohlke, spoiled this momentum.

Based largely on serological studies, Mez and Ziegenspeck produced a “Stammbaum” or “family tree” for the whole plant kingdom. But the results of the Koenigsberg school was unaccepted by another school, that of Gilg and Schurhoff, in Berlin.

As a result of this conflict, there was a decline of serology in Germany. Later, Otto Moritz gave a critical new start to phytoserology in the 1950s, and plant serology has now been established as a valid method in systematics.

Rives, Nelson and Biskeland and Moritz are some of the other early workers, who have made significant contribution to the application of serology to systematics. The role of the school of serology at Rutgers University, New Jersey, headed by Boyden, has also been very significant in the theoretical and technical progress of serotaxonomy in recent years.

Methods Used in Serotaxonomy:

In this method, a crude protein extract of a particular plant taxa (antigen), is injected into the blood stream of an experimental animal, usually a rabbit or a rat, to develop antibodies. In response to the specific antigen injected, a specific antibody is produced in the blood of the animal.

The serum (termed the antiserum) containing the antibody is then collected and made to react in vitro with the antigenic proteins as well as proteins from other related taxa, of which the affinities are in question.

Serological reactions between antibodies and antigenic material results in the formation of a precipitate. This is called precipitin reaction. Kraus (1897) showed that this reaction indicates similarity of antigens. The degree of protein homology is determined by the amount of precipitation and hence is taken as a phylogenetic marker and taxonomic character.

However, according to Moritz and Jensen (1961), crude protein extracts should not be used as antigens, as has been the normal practice in serotaxonomic work. This is because, crude protein extracts contain a large number of proteins, which stimulates the production of a vast range of antibodies, which differ in their specificity and reactivity. Some are produced in abundance while others are hardly detectable.

Further, each protein in turn carries a number of antigenic determinant sites (the portion of the antibody molecule that reacts with a portion of the antigenic material) or epitopes, each of which is capable of eliciting the production of distinct antibody molecules with a specific antibody determinant site or paratope, specific to that epitope.

To overcome these problems and to make highly refined serotaxonomic comparisons possible, Lester (1984) has developed a novel method of pre-absorption i.e., an antibody system induced by immunization with a crude protein extract (the antigen system) of one species is ‘pre-absorbed’ by the antigen system of a second species and then tested with the antigen systems of other species.

The recent development of powerful analytical techniques and instrumentation, has also made possible the use of monoclonal antibodies, i.e. single kinds or species of antibody directed against single epitopes, although such techniques may not be of great importance in taxonomic studies.

Several serological methods are now available.

The most common methods used are as follows:

(a) Immunodiffusion in Agarose Gels:

In this method the antigen-antibody reaction is carried out in gels, mostly of agarose, in petridishes. The antiserum containing antibodies is filled in a well at the centre of the gel and the antigens from related taxa are placed in outer or radial wells.

The antigen and antibody react to produce the insoluble antigen- antibody complex, forming a thin immobile band of precipitin (protein) at equilibrium, which can be visualized either directly or after protein staining for interpretation.

Immunodiffusion in gels can be further of two types:

i. Single radial immunodiffusion:

In this technique, the antigen is usually allowed to diffuse into the gel containing the antiserum.

ii. Ouchterlony – double immunodiffusion:

In this method, both the antigen and antibody are allowed to diffuse into the gel and meet each other.

(b) Rocket Immunoelectrophoresis:

It is a simple, rapid and reliable method in which, rocket-like immunoprecipitate is formed when the desired protein (antigen) is electrophoresed in an agarose gel containing its mono-specific antiserum. A comparison of the height of the peaks of the unknown and standard samples also allows the unknown protein concentration to be determined.

(c) Enzyme-Linked Immunosorbant Assay (ELISA) :

This method is generally used for quantitative estimation of a particular protein in a mixture, but can also be used to study the antigen-antibody reaction. The antibodies against a particular antigen are adsorbed to a solid support, in most cases a polystyrene microtiter plate.

The support after coating with antibody is washed and then the antigen is added, which binds to the adsorbed antibodies. An enzyme-linked antibody molecule called the conjugate is then added, which also binds to the antigen, which is followed by a chromogenic substrate for the enzyme.

The colored product generated is observed for confirmation of antigen-antibody reaction as well as measured for quantitative estimation. The intensity of the color is proportional to the bound enzyme and thus to the amount of the bound antigen.

Role of Serotaxonomy:

Serological studies using crude plant protein extracts have been widely used in elucidating the taxonomy of a wide variety of higher-level taxa and in estimating phylogenetic relationships.

Following are a few examples:

a. A close relationship among the Magnoliidae, Hamamelididae and Comiflorae of the angiosperms has been found, based on comparative serological studies of their major seed proteins. This has refuted the idea of their independent evolution.

b. The homogeneity of the iridoid-producing Comiflorae has been confirmed by serological studies, which has supported the inclusion of the Gentianaceae in it.

c. Based on phytoserological studies, Pickering and Fair brothers (1970) have proposed the classification of the family Umbelliferae into Hydrocotyloideae, Saniculoideae and Apioideae, and Apioideae was found to be more closely related to Saniculoideae than to Hydrocotyloideae.

d. Jensen (1968) discussed the relationships and classification of 20 genera of the Ranunculaceae based on serological evidence, which supports close relationship between Delphinium and Aconitum. Hydrastatis, which is sometime classified with the Berberidaceae has more close serological similarity to the Ranunculaceae than the Berberidaceae.

Trollies, which is regarded as a link between follicular and achenial fruit bearing genera in the family, resembles closely with Adonis.

e. Serological studies have shown that the genus Linodendron is quite distinct from other members of the family Magnoliaceae, and the genera Magnolia and Michelia are most closely related within the family.

f. Serological analysis has been useful in estimating the generic kinship in the Caprifoliaceae and the relationship between the Nymphaeaceae and Nelumbonaceae.

g. Lee and Fair brothers (1978) used serological techniques to study the systematics of Rubiaceae and related families. The quantitative data emphasized the similarity of Rubiaceae to Cornaceae and Caprifoliaceae and the pre-saturation tests revealed similarity with Apocynaceae, Asclepiadaceae and Gentianaceae.

Similarly, two genera under Rubiaceae viz. Asperula and Galium were found to be serologically similar to each other while being the most dissimilar from all other genera of the family.

h. In an attempt to estimate generic interrelationships of the tribe Genisteae of the Leguminoseae, Cristofolini and Chiapella made a serological survey and found that the genus Cytisus (sensu lato), including Chamaecytisus, Sarothamnus and Carothamnus, is a single homogenous unit apart from C. sessilifolius, which in turn is more closely related to Laburnum and Genista than to other species of Cytisus.

Their studies also supported the elevation of the sect. Asterospartum of Genista into a new genus Cytisanthus.

i. Similar studies as in Cytisus was also made in Ulex by Chiapella and Cristofolini and they allied genera of the same tribe.

j. A cross-reactivity serological reaction (i.e., the reaction between an antiserum and any antigenic material other than the antigenic material used in its formation) of the seed proteins of Phaseolus vulgaris with those of other species of the same genus by Kloz, showed a decreasing reaction in the series P. coccineus, P. polyanthus, P. acutifolius, P. lanatus and P. aureus, showing that, P. vulgaris is most closely related to P. coccineus and most distant to P. aureus.

k. The assignment of Phaseolus aureus and P. mungo to the genus Vigna is strongly supported by serological evidence by Chrispeels and Gartner.

l. The prevailing idea of reticulate evolution in the genus Phlox has been corroborated by serological studies of Levin and Schaal.

m. On the basis of double diffusion tests, Gell, Hawkes and Wight (1960) and later Hawkes and Lester compared potato species and showed that Mexican species fell in three groups and Solanum morelliforme is only distantly related to other tuber-bearing solanums.