QBARS - v29n2 Recent Additions to Published Rhododendron Chromosome Numbers

Recent Additions to Published Rhododendron Chromosome Numbers
August E. Kehr, Silver Springs, Maryland
National Program Staff, ARS, USDA, Beltsville, Maryland

Information on chromosome numbers in rhododendrons was summarized in the January 1973 Bulletin of the American Rhododendron Society (4). This article reports on chromosome numbers not included earlier, as follows:

So far as I know the above chromosome counts include the most inclusive reports on the Malesian rhododendrons. All these species counted to date in this group are diploids.
The most interesting report above is that R. occidentale may have a hexaploid form with 78 chromosomes. This form was collected in the Celina Ridge, Nevada County, California. This species has long been considered to be a diploid, as reported by Janaki Ammal, Enoch, and Bridgewater in 1950 (2). Their counts were made on a plant growing in the R. H. S. Gardens at Wisley. In addition Sax in 1930 (8) did a study on two R. occidentale hybrids. The first hybrid ( R. occidentale x R. calendulaceum ) was a triploid with 39 chromosomes, supposedly 13 from the diploid R. occidentale and 26 from the tetraploid R. calendulaceum . The other ( R. occidentale x R. japonicum ) was a diploid with 26 chromosomes, 13 from each diploid parent. From these results it was inferred that R. occidentale was a diploid. Thus the report by Niehaus and Wong (7) differs from the two earlier reports on chromosome counts on R. occidentale . In my opinion there is no disparity between the reports of Niehaus and Wong and the earlier ones. Chromosome counts in species of Rhododendrons have been found to have multiple chromosome series. These have been reported by Janaki Ammal et al (2) as follows:

The data by Niehaus and Wong have three implications if later research verifies them. The first implication deals with the origin of R. occidentale . Plants with compounded chromosome numbers have arisen by hybridization, followed by doubling of chromosome numbers. If no other species of deciduous azalea is found growing in western North America, R. occidentale would represent a single, isolated species growing in its present geographical range. Therefore it would be reasonable to believe that the higher chromosome numbers existing in the present-day species probably represent repeated doubling of an earlier existing diploid species. Polyploids (plants with higher than diploid chromosome number) are usually more recent developments in the evolutionary process.

As a second implication, hybrids between a hexaploid species with 78 chromosomes and diploid species with 26 chromosomes would be expected to be tetraploids with 52 chromosomes. Thus, for example, a cross between R. occidentale and the diploid species, R. bakeri would have 39 chromosomes from R. occidentale and 13 chromosomes from R. bakeri , or a total of 52 chromosomes.

R. occidentale has been one of the several parents of the Exbury-Knaphill-Slocock-Ilam group of deciduous azaleas. During the intercrossing of the various original hybrids a hexaploid form of R. occidentale would likely have contributed its higher chromosome number to these hybrids, increasing their resultant chromosome makeup. These offspring with higher chromosome would probably be selected for their larger flower sizes in the development of this group of azaleas.

For some time I have believed that higher chromosome numbers may have accounted for the large size of the Exbury-Knaphill-Slocock-Ilam azaleas and that there were many polyploids among these plants. The finding that at least one form of R. occidentale is a hexaploid substantiates this theory, particularly if the parents used by the first European hybridizers came from the region in which the hexaploid is reported. Can it be that the plants of R. occidentale used in the Exbury-Knaphill-Slocock-Ilam azaleas came from this area? Dr. Frank Mossman informs us that the species is represented in that location by small flowered specimens. Of course another parent which could have contributed to increased chromosome numbers in these azaleas is R. calendulaceum , a tetraploid.

A third implication arises. Dr. Frank Mossman and Britt Smith have discovered and described the unusual range of variability in R. occidentale . The tremendous diversity was puzzling and hard to explain, as long as R. occidentale was believed to be a diploid species throughout its geographical range. The new finding may explain the variability of forms they have found. These new data imply that a range of chromosome numbers are found in the species, from diploid (or 2n) to hexaploid (or 6n). A hexaploid form of R. occidentale would be unique among the deciduous azaleas of the world.

Members of the American Rhododendron Society contemplating counting chromosome, might wish to follow the methods and materials used in making the above counts. In the two reports giving this information (1, 5), buds, root tips or both were fixed in 1 part glacial acetic acid to 3 parts 95 per cent ethyl alcohol for 24 to 48 hours, after which they were transferred to 95 per cent ethyl alcohol. The material was squashed and stained for counting in 1 per cent acetocarmine.  For full details on this technique obtain from your library a copy of the book "Plant Microtechnique" by D. A. Johansen published by McGraw Hill. For those requesting it, we can provide a brief summary of this method.

LITERATURE CITED

1. Hsu. C. C. 1968. Preliminary chromosome studies on the vascular plants of Taiwan (II). Taiwania 14: 11-27.
2. Janaki-Ammal, E. K., 1. C. Enoch. and M. Bridgewater. 1950. Chromosome numbers in species of Rhododendron . Rhod. Yearbook, Royal Hort. Sec. 5: 92-98.
3. Jones. K., and Christine Brighton. 1972. Chromosome numbers of tropical Rhododendrons. Kew Bulletin 26(3): 559-561
4. Kehr, A. E. 1973. Some questions and answers about tetraploids and colchicine. Quart. Bull., Am. Rhod. Soc. 27(1):20-23.
5. Mehra, P. N., and K. S. Bawa. 1969. Chromosomal evolution in tropical hardwoods. Evolution 23:466-481.
6. Mossman, F. 1972. With camera, white umbrella and tin pants in R. occidentale heartland. Quart. Bull., Am. Rhod. Soc. 26(4):218-222.
7. Niehaus, T., and L. Wong, Jr. 1971. In IOPB chromosome number reports XXXVI. Taxon 20 (2/3):349-356.
8. Sax, Karl. 1930. Chromosome stability in the genus Rhododendron , Amer. Jour. But. 17:247-251.
9. 9. Zhukova, P. G. 1967. Chromosome numbers in some species of plants of the north-eastern part of the U.S.S.R. II (In Russian). Bot. Zhur. 52:983-987.

INTERPRETIVE SUMMARY

Literature on recent chromosome numbers in Rhododendrons is reviewed. The reports include the first numbers for the Malesian group. The evolutionary and genetic implications of the finding that R. occidentale is a hexaploid were discussed.