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{{multiple issues|
{{essay-like|date=May 2014}}
{{more citations needed|date=May 2014}}
{{Orphan|date=April 2016}}
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The '''Cytodeme''' is the total assembly of organisms that use an identical suite of chromosomes to carry their genes.{{citation needed|date=May 2014}} The term was first printed in the 1950s in a book by Heslop-Harrison.<ref name="Heslop-Harrison">Heslop-Harrison,1953.''New Concepts in Flowering Plant Taxonomy'' Heineman, London</ref> Discussing the ''Deme Terminology''<ref name="Gilmour & Gregor">Gilmour & Gregor, 1939. "Demes:a suggested new terminology" ''Nature, London'' '''144''':333-334</ref> - he continued ''"cytodeme, a population differing in some distinctive cytological feature from others."'' In most cases the suite is composed of several pairs of [[homologous chromosome]]s with or without a pair of [[sex chromosome]]s. Since the only acceptable proof of the identity ([[homology (biology)|homology]]) of chromosomes lies in their ability to pair fully from end to end during [[meiosis]] it follows that:

#In asexual taxa membership of a cytodeme can only be presumed on such evidence as visual similarity of chromosomal ideotype, but never proved.
#In sexual taxa all of the members of one cytodeme are fully intercompatible (within the limits of sex and other genetical constraints like self-incompatibility mechanisms) yielding the fertile progeny which proves their membership.

As a general rule for most species all of its members are of the same cytodeme excepting only the infrequent [[aneuploid]] aberrants. However some species are known to include several chromosome races which must necessarily belong to different cytodemes. Although it is not intrinsic in the formal definition it is a matter of common observation that members of different cytodemes are essentially cross-incompatible and any hybrids that do arise are usually highly infertile.

==Significance==
The current surge of interest in cytodemes stems from the realisation that membership of the cytodeme is not necessarily restricted to the members of one species: frequently two or more species are in fact of the same cytodeme. Although fully intercompatible and yielding fertile hybrids when they do cross these species never or rarely cross-breed in the natural environment because they are spatially isolated, geographically and/or ecologically. Typically the species involved are members of the same genus. There are now a substantial number of cases in which two or more species of the same cytodeme are clearly different and even in different genera. Thus all five [[diploid]] species recognised in the genus ''[[Brassicella]]'' and both species recognised in the genus ''[[Hutera]]'' (7spp. in total) are in the same cytodeme.<ref name="Harberd">Harberd,1972. "A contribution to the cyto-taxonomy of ''Brassica''(Cruciferae) and its allies." ''Botanical Journal of the Linnean Society''.'''65''':1-23</ref> Perhaps the most extreme case on record concerns the wild grass, [[teosinte]], ''[[Euchlaena mexicana]]'' and the strikingly different Maize, or Indian Corn, ''[[Zea mays]]'', both 2n=20, fully interfertile and yielding fertile [[hybrid (genetics)|hybrids]].<ref name="Beadle">Beadle, 1980. "The Ancestry of Corn". ''Scientific American'','''242''':112-119</ref> There is perhaps a remote possibility that two such very different species could have evolved independently from distinct sources and converged in their chromosomal ideotype until they became members of the same cytodeme (including the capacity to cross-breed). It is more likely, though, that the cytodeme arose first complete with its suite of chromosomes and breeding patterns all intact and then, remaining constant in its fundamentals, it diversified into species sometimes so different as to merit generic distinction. Thus, in what may be termed the ''cytodeme adjunct'' to Darwinian theory, evolution becomes a two-stage process - first, the establishment of distinct cytodemes reproductively isolated both from one another and from all previously existing cytodemes; second, diversification within cytodeme to yield taxonomically recognisable (but not reproductively isolated) species.

Whereas the second stage is [[Natural Selection]] as expounded by [[Charles Darwin|Darwin]],<ref name="Darwin">Darwin,1859.''On the Origin of Species by Means of Natural Selection''. John Murray, London</ref> the first stage is not necessarily Darwinian{{citation needed|date=May 2014}}. In the special case of [[polyploidy]] it is known that the first stage is not Darwinian. Doubling the chromosome number of the [[sterility (physiology)|sterile]] hybrid between two diploid cytodemes yields an [[allotetraploid]] which is both fertile within its bounds and essentially incompatible with all previous life forms.

As yet no mechanism is known which could account for the origin of new diploid cytodemes - nor is there any proof that there are any diploid cytodemes of recent origin. Indeed, the evidence is that cytodemes are remarkably long lived. Presumably the ''[[Platanus]]'' cytodeme existed on [[Pangaea]] before that land fragmented. The present day forms of ''Platanus'' - ''[[Platanus orientalis|P. orientalis]]'' in Europe and ''[[Platanus occidentalis|P. occidentalis]]'' in America - are fully intercompatible when brought together artificially in Botanic Gardens yielding fertile hybrids like the London plane,<ref name="Santamour">Santamour, 1972. "Interspecific hybrids in ''Platanus''." ''Forest Science''.'''18''':236-239.</ref> establishing not only that they are in the same cytodeme as one another but also that they are both in the same cytodeme as their common ancestor of approximately 200 million years ago.

== References ==

{{Reflist}}

==Further reading==
*Lannér, Carita. "Genetic relationships within the Brassica oleracea cytodeme: comparison of molecular marker systems." Acta Universitatis Agriculturae Sueciae. Agraria (Sweden) (1997).
*LIU, Yao, and Wei QIAN. "Crossability among Variant Types of Brassica oleracea Cytodeme." Journal of Plant Genetic Resources 14.2 (2013): 342-346.
*{{cite journal|last1=Warwick|first1=S.I.|last2=Black|first2=L.D.|title=Molecular systematics of Brassica and allied genera (Subtribe Brassicinae, Brassiceae) ?chloroplast genome and cytodeme congruence|journal=Theoretical and Applied Genetics|volume=82|issue=1|year=1991|pages=81–92 |issn=0040-5752|doi=10.1007/BF00231281|pmid=24212864 |s2cid=10758800 }}
*Gómez-Campo, César, ed. Biology of Brassica coenospecies. Vol. 4. Elsevier, 1999.
*Kowalski, Stanley P., et al. "Comparative mapping of Arabidopsis thaliana and Brassica oleracea chromosomes reveals islands of conserved organization."Genetics 138.2 (1994): 499-510.
*Mei, J., et al. "Genetic investigation of the origination of allopolyploid with virtually synthesized lines: application to the C subgenome of Brassica napus."Heredity 106.6 (2011): 955-961.


[[Category:Genetics]]

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