CHROMOSOME NUMBERS OF SOME TROPICAL RHODODENDRONS ( SECTION VIREYA )

Chromosome counts for 27 species plus one intersubsectional hybrid are presented from all seven subsections of section Vireya in the genus Rhododendron . Twenty-three are new, and ﬁve conﬁrm previously published counts. The plants originate from a wide geographical and altitudinal range; all have a uniform somatic number of 2n = 26.


INTRODUCTION
Rhododendron L. (Ericaceae) section Vireya (Blume) Copel.f. is the largest section in the genus, being a group of 303 species (Argent et al., 1996), distributed from the Himalayas and South China through the South-East Asian archipelago to the Solomon Islands and the northern parts of Australia.There is considerable morphological diversity in the section, and seven subsections have been described based on floral and scale characters (Sleumer, 1966).Current taxonomic studies have indicated that floral characters are highly plastic and subject to considerable selection pressures because of their role in pollination biology (Stevens, 1976;Argent et al., 1988).Other characters such as floral bracts and fruits have been sought, and these have indicated that the current infra-sectional classification is in need of revision (Argent, 1989).Despite the high level of morphological diversity there is considerable hybridization reported in Vireya rhododendrons (Rouse & Williams, 1989;Williams & Rouse, 1997) suggesting that genetic divergence in terms of reproductive barriers between species and as well as between subsections is low.
Chromosome numbers for many species of Rhododendron were initially investigated nearly 70 years ago, and relatively recently the most extensive survey was that of Janaki Ammal et al. (1950).This author also summarized the information on polyploidy in relation to geographical and altitudinal distribution and classification of the genus (Janaki Ammal, 1950).These studies, which sampled 368 taxa, indicated that polyploids occurred frequently among lepidote (scaly-sleaved ) rhododendrons, but not among the elepidotes (those without scales).Polyploid species were found in a relatively restricted area of the Sino-Himalayan region, whereas diploids (2n= 26) were far more widely distributed over the full geographic range of sect.Rhododendron.Only two species among these non-Vireya rhododendrons were reported to have n=12 (i.e.2n=24), namely, R. wallichii Hook.f.(as R. campanulatum var.wallichii Hook.f.) and R. grande Wight (Mehra, 1976).These two species are not closely related, and it is a surprise to find that they share a common, deviant chromosome number (Chamberlain, pers. comm.).Clearly, confirmation of these counts is highly desirable.
Four species in sect.Vireya were counted by Janaki Ammal et al . (1950) and all were diploid (2n=26).The most significant previous investigation of the cytology of tropical Rhododendrons, all belonging to sect.Vireya, was that of Jones & Brighton (1972) who reported 33 counts.They found 32 diploids (2n=26) and one individual with 2n=30, presumed to be an aneuploid.Although several subsections were included in this study the geographical range of the section represented was rather limited, as the majority of individuals counted were from New Guinea.In addition, 12 of the counts, including the aneuploid, were of unnamed individuals, and there is doubt as to the accuracy of names of some of the others.We know of no further recent chromosome counts reported for Vireya rhododendrons in the literature.It was decided to further investigate whether or not polyploidy or some other cytological variation occurs in this section.

MATERIALS AND METHODS
The Royal Botanic Garden Edinburgh (RBGE) has an unrivalled living collection of half the Rhododendron species from section Vireya, representing all subsections and a very wide altitudinal (500-2500m) and geographic range (Fig. 1).This collection provided the opportunity to sample further in the section using reliably named living material of known provenance.
Roots were taken from cuttings and pretreated for five hours in 0.002M 8-hydroxyquinoline (OQ) kept in the dark at 20°C.Roots were then fixed in 351 absolute alcohol5glacial acetic acid and stored at 4°C until needed.Prior to squashing fixed material was softened in a 50550 mixture of 4% pectinase and 4% cellulase for 30min at 37°C.Root tips were then dissected in 45% acetic acid and squashed in lacto-propionic orcein.All observations were made using phase contrast microscopy.This protocol was found to be reliable for all the material examined but some experimentation during later stages of this study was made using a saturated solution of oryzalin as a pretreatment for five hours at 20°C in the dark.This was found to be an effective pretreatment although the chromosomes tended to be more contracted and clustered than those treated with OQ.
Vouchers for all examined species are lodged in the herbarium of the RBGE ( E ).

RESULTS AND DISCUSSION
Counts were made of 28 accessions representing 27 species and one intersubsectional hybrid (R. caliginis Kores×vitis-idaea Sleumer).one pair of satellited chromosomes was often present (e.g.Fig. 2f&h).Some variation was observed in chromosome size between species, but this needs confirmation based on a more specific study.
Among the four species of section Vireya surveyed by Janaki Ammal et al. (1950) was included R. lochiae (as R. lochae), whose number of 2n=26 is confirmed in the present study and also by Jones & Brighton (1972).This species is found in Australia, one of the geographical extremes of the section.The counts made by Jones & Brighton (1972) on four other species, namely, R.? beyerinckianum, R.? aequabile, R. orbiculatum and R. commonae were also concordant with our observations on different accessions of the same taxa, although we have not been able to trace voucher specimens to check their taxonomic identity.
These results show that chromosomal diversity in Rhododendron sect.Vireya is low even across a wide geographical and taxonomic sample and support the hypothesis that high morphological diversity in the group is not reflected in much cytological diversity, at least not at the gross chromosomal level.Chromosome counts now available for about 60 taxa are almost uniformly 2n=26.To date, only one case of an aneuploid number of 2n=30 has been reported (Jones & Brighton, 1972).There is as yet no evidence of polyploidy in sect.Vireya, in contrast to sect.Rhododendron where polyploidy occurs among one third of cytologically examined lepidote species, the degree of polyploidy ranging from triploids (2n=39) to dodecaploids (2n=156) (Janaki Ammal, 1950).Enormous species diversification has clearly occurred at the diploid level, as is the case among elepidote species of sect.Rhododendron, which are also all diploid as far as is known.
TA BL E 1. Chromosome numbers of Rhododendron (section Vireya)