Basic Chromosome number in Orchidaceae

Hoffmann (1930) , Duncan (1959) and Blumenschein (1960) have suggested that the probable basic number for the family is 20, by virtue of the fact that this is the number possessed by the majority of the orchid genera investigated . The number 20 is , no doubt , very prevalent among orchids ; but the number 19 is even more prevalent than 20 . There are about 280 species with 19 or its multiples as their n number , while the number 20 or its multiples are seen only in 274 species . Besides , the number 20 appears to have different origins in different groups of orchids. Miduno (1938) has shown how in the subtribe Limodorinae Benth., which is considered to be the most primitive of all mondandrous orchids, the number 20 is derived from 16, by fragmentation of chromosomes. The chromosome numbers in Cephalanthera and Epipactis of this subtribe range as 32, 34, 36 , 28 and 40. In the species investigated by him, Miduno found 6 very large chromosomes exactly as in the allied genus Listera, plus an assortment of small chromosomes , which are to be assumed as products of fragmentation.

Cephalanthera schizuoi -6L + 26 s = 32 (2n)

C.erecta - 6L + 28 s = 34 (2n)

Epipactis sayekiana - 6 L + 2 Lt = 40 (2n) = 32s

E. thunbergii - 6L + 34 s = 40 (2n)

* L = long ; s= small ; Lt= satellited .

miduno considered the probable basic chromosome number here as 8 from which 16 and later 20 are derived .

Garay ( 1960 ) is of the opinion that each group of orchids may have its own basic number . This appears to be moer plausible , particularly in view of the postulated ployphyletic origin of the family. Very often we find more than one basic chromosome number wihtin the same tribe . Taking Orchideae for example , the genera Habenaria and Platanthera appear to have evolved their species upon the basic number 7( Habenaria oldhamii, H . Sagittifera - 2n= 28, H . chlorantha , Platanthera bifolia - 2n = 42; P. Obtusata , P. hyperborea -2n = 63 ; p. oligantha -2n =126 ). In Orchideae itself chromosome numbers seem to have been derived from 6 as well (Serapias longipetala 2n= 24 ; S. vomeracea , Ophrys, Himantoglossum , etc. 2n= 36). Of the 10 subtribes included in Neottieae , Limodorinae , Neottiinae and Spiranthinae are comparatively well investigated . As mentioned previously , Limodorinae have a probable basic number 8 , which is true also for Listerinae with which it is closely related. But the subtribe Spiranthinae presents a different picture . Chromosomes in this subtribe are as given eblow:

Anoectochilus -n = 20 Tanaka , 1965

Haemaria -n= 22 T anaka , 1965

Hataeria -n - 20, 21 Tanaka , 1965

Goodyera - n = 14, 15, 16 , 21 & 22 Miduno , 1939 ; Love and Love , 1942 ; Richardson , 1935 ; Tanaka , 1965.

Zeuxine -n = 10, 21 & 22 Seshagiriah , 1934; Mehra and Pal , 1960 ; Vatsala , 1964 ; Tanaka , 1965.

Spiranthes - n= 15, 30 Pace, 1914 ; Miduno , 1939 ; Hagerup , 1944 a ; Vij and Vohra , 1971 ; Tanada , 1965 .

The lowest n number here is 10 and the majority of numbers are multiples of 5 . So here the basic number is 5 , the other numbers are being derived from it at various ploidy levels . Thus within the same tribe Neottieae we find two basic numbers 5 and 8 . Any , for the majority of orchids , it would not be possible to tracea line of evolution from a single basic number because of the factor of hybridisation also involved in the process. As Garay (1960) has pointed out , the pattern of evolution in Orchidaceae to trace is reticulate rather than linear and as such , is a much more complex process to trace . The family must have evolved from a series of basic chromosome numbers , the process involving euploidy , aneuploidy and above all hybridisation.

The n numbers in orchids range from 10 to 21 with a few species having 23 , 27 and 29 . Of these the prevalent numbers are 19 , 20 and 21 in that order . In tribe Neottieae the numbers are more or less generalised with no peaks. In Orchideae there is a sharp peak at number 21 (53 species) and a lesser one at 20 (26 species ). As has been pointed out earlier , the progress of evolution in Orchidaceae is characterised by a stabilisation in the number of chromosomes . In the advanced tribe Epidendreae the number has settled down to either 19 or 20 in the majority of species, more than 70 % of the genera showing either or both these numbers. But a survey of the chromosome numbers in Asiatic and American tropics reveal certain interesting facts . While both the numbers 19 and 20 are prevalent in the tropical Asiatic species , in the American tropics the 20 -chromosomed species are the predominant ones, with only two species of Oncidium showing 19 as the haploid number . The table given below will help to clarify the situation :

The obvious conclusion which could be drawn from the above data is that the number 20 is more generalised and ancient of the two , being prevalent in both hemispheres and the number 19 is probably derived from it. The fact that in Sarcanthinae , the most advanced of Eastern Orchids, 19 is the most prevalent , supports this assumption .