Variation of Chromosome numbers and speciation in Orchidaceae

As is generally understood, speciation can be effected by polyploidy (both euploidy), hybridisation , chromosome repatterning and gene mutations. Duncan (1959) categorised the chromosome number variations among orchids under four heads :

1. All species in the genus showing the same chromosome number , eg. Ophrys .

2. Species in a genus differ by some having an additional number , eg. Dendrobium.

3. Species in a genus forming an aneuploid series , eg. Paphiopediulm.

4. Species in a genus forming an euploid series , eg. Dactylorchis . Such a generalisation of the chromosome number variation would have been same in all the tribes. A general survey of the chromosome numbers so far known, shows that this is not the case that each tribe has its own pattern of evolution

Taken as a whole, the most prevalent form of speciation in orchids appears to be that which has occurred without any change in the number of chromosomes . This type of speciation occurs in more than three-fourths of the genera studied . The incidence of aneuploidy is evident in a little less than half of the genera. Euploidy appears to be the least favoured form of speciation in orchids, less than one-third of the genera investigated having produced species by this method .

When we take into considertation the tribes separately , again Type III , ie . speciation without variation in chromosome number , seems to be the most prevalent , except in the very small tribe Cypripedieae , where aneuploidy is predominent . The percentage of incidence of Type III variation increases steadily with the advancement of the evolutionary status of the tribes - from Cypripedieae ,where it is only 66. 67% to Epidendreae , where it has risen to 80. 46 % . In the subtribe Sarcanthinae , the chromosome numbers have settled to remarkable degree of constancy . Except in the aberrant Angraecoids and their allies , the chromosome number here is invariably 38 or its multiples. This tendency towards constancy of chromosome numbers is evident in other hand probably less advanced subtribes of the Epidendreae . The immensely vast genus Dendrobium is an example . This genus is the second largest Orhidaceae and possesses upwards of 1500 species (Hawkes , 1970) and has a wide distribution in the Eastern hemisphere , extending from Korea and Japan through the Indo -Burmese and the Malaysian region to Australia , New Zealand and to certain Pacific groups of Islands. Morphological variations in the vegetative and floral characters of the species of this genus are so diverse , that Holttum (1953) suggested that the sections of Dendrobium are biologically far more distinct than many orchid genera in other subtribes . Kamemoto , Shindo and Kosaki (1964) studying the breeding behaviour of several inter -and intra - sectional hybrid of this genus ,came to the conclusion that the genetic isolation apparent at specific level in Dendrobium is even greater than if is at generic level in the subtribe Sarcanthinae . Yet, within this giant of a genus ,almost equal in number of species to the whole subtribe Sarcanthinae , only two chromosome numbers , 38 and 40 (or their multiples) are known. The related genus Bulbophyllum , with nearly 2000 species and enjoy a wider distribution thatn Dendrobium presents the very same picture , possessing only two chromosome numbers 38 and 40. In the tropical Asiatic Cymbidium , Coelogyne and Polustachya and the tropical American Cattleya and its allies , Laelia and Brassavola, the chromosome number has settled down to a constant 40 . In genera like Calanthe and Epidendrum , on which considerable amout of work has been done , the great majority of species have 40 chromosomes .

Since changes in number of chromosomes between species are not so prevalent in orchids , it is to be assumed that changes within the complement are resoponsible for directing the course of evolution in the family . This could be brought about by hybridisation , cryptic structural changes in the chromosomes and gene mutations . Due to certain morphological , genetical and embryological peculiarities in orchids , hybridisation is easily facilitated and the accompanying meiotic irregularities and consequent sterility of gametes avoided to a considerably greater extent than in other plant families , with the result that hybridisation has become a continuing and major factor in the process of evolution in Orchidaceae , providing a very speedy form of evolving new species .

The incidence of aneuploidy , which seems to have played the second important role in speciation in Orchidaceae , is varied in the different tribes. In Cypripedieae , all the three investigated genera show this type of variation and hence its incidence is 100% . The n number here varies from 10-21 in an unbroken series. In the three remaining tribes., Neottieae has the greatest measure of aneuploidy with 46. 67% and Epidendreae the least with 39.17% . Orchideae coming in between . The tropical American subtribe Oncidiinae provide an excellent example od speciation brought about by aneuploidy . Between the genera Odontoglossum and Oncidium , the chromosome numbers(n) of the species range from 13 to 22 and 24 in an unbroken series (Dodson 1957 a,c; Sinoto , 1962 ; Blumenschein , 1960 ; Chardard , 1963 ). Lesser series of aneuploids are also reported in Eulophia where the n number ranges from 17, 20, 22, 23, 28, 30, 31, 33, 41 and 48 (ar- Rushdi, 1971; Vatsala , 1964; Ninan et al. unpublished ) Liparis , where n number ranges as 14, 15, 16, 18 , 19 and 21 (Miduno , 1940; Mehra and Pal, 1960; Vatsala , 1964; Tanaka , 1965; Arora , 1971) Goodyera , where n number ranges as 14,15, 16, 21 and 22 (Miduno , 1939; Tanaka , 1965) and Angraecum , where n number ranges as 19, 20 , 21 and 25 (Eftimiu-Heim, 1941; Chardard, 1963). The small genus Tainia with only 3 species investigated , nevertheless, shows very variable chromosome numbers n=15, 18 and 20 (Tanaka 1965; sharma and Chaterji , 1966 and Ninan et al. unpublished).

Aneuploidy may result from occasional non-disjunction of chromosomes during anaphase or by the partial return to the original chromosome number in triploids and pentaploids as is shown to be the case in some of the horticultural varieties of Vanda by Kamemoto (1958) . Duncan (1948) has reported regular somatic non-disjunction of chromosomes with subsequent production of tissues with variable number of chromosomes in Paphiopedilum . Chardard (1963) has reported cells with variable number of chromosomes in the same tissue in 35 out of the 107 species of orchids which he has investigated . Sharma and Chatterji (1966) also have observed widely varying chromosome numbers in the same species , eg. Coelogyne corymbosa 2n=38 and sometimes 22; Calanthe brevicornu 2n=38 and sometimes 57 etc. Several polysomics have been observed in Eulophia euglossa Polystachya rhodoptera by ar-Rushdi (1971). According to Barber (1942) ,the rate of incidence of aneuploidy is enhanced in orchids and pollen mitosis sets in all the four cells simultaneously . Thus the physiological control over the process is uniform on all the four cells, whether they are dificent or otherwise , at all times. This enables all the four cells to survive and function .

Speciation in such genera as Paphiopedilum , Goodyera , Eulophia and Oncidium , which have chromosome numbers in aneuploid series , also involves euploidy to a great extent. Here it is to be assumed that aneuploidy at levels of ploidy higher than diploidy , have arisen as a result of the mild irregularities in meiotic processes and fusion of gametes with altered chromosome numbers.

The cases of euploidy are rather scarce in orchids. Most often it is found to occur in association with hybridisation . This is most prevalent in the tribe Orchidieae . Where 50 % of the genera show this type of chromosome number variation . The Dactylorchids with 40, 80 and 120 chromosomes (Vermulen , 1947 , 1948 , 1949) and the Habenaria - Platanthera complex with 1940b; Himphrey , 1933 etc) provide excellent examples of speciation by euploid polyploidy . Chennaveeriah and Jorapur (1966) have reported a series of polyploids in the genus Nervilia -N. infundibulifolia 2n=54; N. plicata 2n=108; N. monantha 2n= 144 . Apart from these , its sporadic occurrence is also reported in genera like Vanilla, Coelogyne, Vanda , Doritis etc. Out of the 15 species of Vanilla so far investigated , 12 have 32 chromosomes (Eftimiu- Heim, 1954 ; Martin , 1963; Hoffmann , 1930) and one species has 36 chromosomes (Krupko et al. 1954); but two species V. haapape (Tonhier, 1951)and V. wightinana (Vatsala , 1964)are polyploids with 64 chromosomes. Similarly the genus Coelogyne has 17 species investigated , out of which 15 have 40 chromosomes (Hoffmann, 1929; Mehra and Pal, 1960); but one species ,C. ovalis, has the doubled number 80 (Vatsala 1964). We know the chromosome numbers in 22 species of Vanda , out of which 19 are diploids with 38 chromosomes (Woodard 1952; Storey , 1952; Tanaka and Kamemoto, 1961; Vatsala , 1964; sharma and Chatterji , 1966;Chardard , 1963). V. Tricuspidata, V. tricolor (story, 1952) and the south Indian strain of V. spathulata (Vatsala , 1964) are tetraploids with 76 chromosomes . The V. spathulata examined in Hawaii (Storey , 1952; Storey , Shindo and Kamemoto , 1963) is a hexaploid with 114 chromosomes.

A few cases of autopolyploidy also have been observed in orchids. Examples are given below:

The diploid and triploid varieties of Calanthe masuca investigated by us , were both collected from the same locality -Kannikatty in the Western Ghats of Tamil Nadu. Meiosis in the diploid was highly irregular with many bridges and laggards on the anaphases plates . Among the pollen , restitution monads, diads and triads were liberally present along with regular tetrads . Since the triploid has come from the same locality as the diploid in this case, it may be inferred that triploid is the result of fusion of an unreduced gamete with a haploid one. In the meiosis in the triploid the majority of configurations at metaphase I wer of trivalents , thus providing the autopolyspoid nature of the plant .

Duncan (1959) has described how post-maturation changes in gametes , which have to wait for an unusually long time for fertilization to take place, can lead to polyploidy . This happens in cases where there is lack of conformity in the time of maturation between the male and female gametophytes . In many cases this leads to sterlity . But occasionally , the female gametophyte, which meatures first, waits for the pollen tube to grow and reach the necessary length in order to fertilise it . While thus waiting , post-maturation changes take place in the female gametophyte , most common of which is teh fusion of the egg nucleus with its adjacent nuceli, giving rise to gametes with unreduced number of chromosomes. As example , Duncan cites crosses between species of Paphiopedilum ,which have short and long fruit -development cycle.

Paphiopedilum xOlivia (p. tonsum x P. niveum)

2n =(17x 2)+ 13 =47

Paphiopedilum X chapmannii 'Magnificum' (P. curtsii X P. bellatulum)

2n = (18 X 2) + 13 = 49

Polyploidy in orchids may also result from polyspermy. According to Hagerup (1944 b) in Orchis maculata L. Sensu lato , polyploidy has arisen due to fertilization by more than one sperm.

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