Hybridisation

Hybridisation is the most facinating of the many branches of Orchidology. In no other branch of the plant kingdom do we find such uninhibited intermingling of genomes, not only at the specific level but also at the generic level . This has contributed greatly to the impressive rate of evolution in the family ,which turns out new species and carieties at a very rapid rate . While turns out new speceis and varieties at a very rapid rate. While Compositae and Gramineae are the most successful families on earth from the point of view of the population number and distribution , Orchidaceae easily exceeds them in the number of species and varieties produced and their evolutionary status.

Evidences of hybridisation occurring in the natural state among wild members of the family have been noticed from the earliest days of orchid collection and cultivation . In almost all genera which are horticulturally important , and in which wide collections have been made from different parts of the world , instances of natural hybridisation have been recognised by Lindley in 1853 as a natural hybrid between P. aphrodite and P. rosea . This was proved true when Seden , who followed P. aphrodite and P. rosea . This was proved were several other examples of a similar kind . Masdevallia splendida and M. parlatoreana were recognised as hybrids of M. veitchiana and M. barloeana ,which exist side by side in the Andes of Peru.

Whole complex of natural hybrids has been traced among the Mexican and Columbian Odontoglossums (Veitch , 1881 ). Here hybridisation is so prevalent between species like O. crispum , O . odoratum , O . heterocarpum, O. lindleyanum , O. prescatoria , O. triumphans etc., that it is difficult to draw a definite line of separation between species . At least five different groups of hybrids can be distinguished in this complex:

1.the crispo-odoratum group

2. the crispo-lindleyanum group

3. the crispo-luteopurpureum group

4. the odorato- luteopurpureum group

5. the triumphante- prescatoria group

Natural intespecific hybrids have been reported also in other genera like Cattleya , Laelia , Miltonia , Oncidium etc.

There are also records of intergeneric hybrids evolved under natural conditions. Cattleya guatemalensis , collected from Guatemala in 1861 by Skninner was later shown to be a natural bybrid between Epidendrum aurantiacum and Cattleya skinneri. It later came to be known under the generic name of Epicattleya. Tatum (1930) reported that in Santa Catarina , a small island near the coast of Brazil ,where Laelia purpurata , Cattleya leopoldii and Cattleya intermedia exist side by side, three distinct natural hybrids have resulted due to free interbreeding.

C. intermedia x C. leopoldii = C. intricata

L. purpurata x C. leopoldii = Laeliocattleya elegans

L purpurata x C. intermedia = Lc. scilleriana

The significance of this occurrence of prolific natural hybridisation in orchids is expressed more in the evolutionary process in the family than in any other aspect . Lenz and Wimber (1959) have pointed that hybridisation followed by backcrossing which is common among juxataposed genera leads to the restriction in even 'wide crosses' in this family is an interesting problem to look into . As a rule ,when two different species are brought together for the purposes of hybridisation , the main difficulties encountered are the lack of compatibility either between the parental genomes or between the embryo and the mother tissue . Lenz and Wimber (1959) give plausible reasons to explain how the orchids have succeessfully surmounted these obstacles.

1. The lack of endosperm in orchids is conducive to the survival of a larger number of hybrid embryos than in other families .

2. Evolution in Orchidaceae is far too rapid for the effective establishment of a successful barrier.

Usually in hybrid zygotes which carry an incompatibility factor between parental genomes it has been observed that the embryo grows to a certain extent and a mass of cell are formed . But growth does not proceed beyond this stage and eventually the embryo degenerates . This has been explained as due to the incompatibility between the maternal tissue and the foreign element brought in by the male gamete . But in Orchidaceae , the embryos , do not get differentiated and growth stops at the stage when it is an undifferentiated mass of cells. In this condition the very small seeds are released from the fruit and scattered by wind . Further development of the embryo in nature is dependent on contact with a fungal mycelium which supplies the necessary nourishment for its growth . Thsu due to the non-endospermic conditions ,the orchid embryo, hybrid or otherwise , could successfully escape the crucial stage in its life , namely , adaptation to the mother tissue .

Due to the surprising amount of compatibility between parental genomes in species hybrids in orchids, meiosis is remarkably regular in most of these and consequently sterility is low. This compatibility between genomes is explained by Lenz and Wimber as due to the absence of an effective barrier.

The efficiency of the sterility barrier among the orchids is also hampered by the production of the innumerable number of seeds , at least some of which manage to survive.