The chromosomes of Chortolirion and Poellnitzia (Asphodelaceae: Alooideae)

The somatic chromosomes of Chortolirion angolense (Baker) A. Berger and Poellnitzia rubriflora (L. Bol.) Uitewaal have been studied. Both taxa are monotypic genera in the subfamily Alooideae of the Asphodelaceae. Prior to this study Chor­ tolirion had not been cytologically examined, while karyograms and idiograms have never been presented for Poellnitzia. Chortolirion and Poellnitzia are diploid with 2n = 14 chromosomes and with a bimodal karyotype typical of the entire subfamily, comprising four pairs of long chromosomes and three pairs of short chromosomes. For Poellnitzia minute structural differences in karyotype morphology were noted between observed and published data, especially with regard to the presence of satellites. daar by Poellnitzia fyn. strukturele verskille in kariotipe-morfologie tussen waargenome en gepubliseerde data.


INTRODUCTION
The subfamily Alooideae of the Asphodelaceae (sensu Dahlgren et al. 1985) includes about seven genera and more than 450 species. Most Alooideae taxa are found in subSaharan Africa, with a marked concentration of species and genera in southern Africa. All species of Alooideae are petaloid, succulent-leaved, rosulate or distichous perennials. They differ in size from miniatures barely 10 mm high (Haworthia parksiana Von Poellnitz) to trees of massive bulk up to 20 m tall (Aloe bainesii Thiselton-Dyer). In spite of this notable morphological variation the Alooideae is reasonably homogeneous in an evolutionary sense, unifying characters being the widespread occur rence of secondary thickening growth, leaf succulence, usually tubular petaline flowers and fusion of the perianth segments. Furthermore, in the entire subfamily the basic diploid karyotype (2n = 14; four pairs of long chromo somes and three pairs much shorter) is only very rarely altered (Brandham 1969).
The majority of species of Alooideae has been investi gated cytologically. These studies were initiated early in the 20th century and have resulted in an extensive bibliography on the cytology of this group (for reviews see Muller 1941; Riley 1959a.b.c; Brandham 1971Brandham , 1983Riley & Majumdar 1979

C.
angolense (Baker) A. Berger is a perennial, deci duous herb with a subterranean bulb and is widely distributed in the summer rainfall region of southern Africa. With short, annual, succulent shoots arising from the bulb in early spring. C. angolense presents a com bination of geophytic and succulent habits ( Figure 1). P. rubriflora (L. Bol.) Uitewaal is a low-growing, caulescent, succulent herb. Stems are up to 250 mm long and densely leaved. The ovate leaves are pungentacuminate and up to 40 mm long ( Figure 2). P. rubriflora has a restricted distribution in the south-western Cape Province of South Africa. This region receives its pre cipitation mainly during the w inter months.  Plants for cytological study were grown in the green house of the Department of Botany, University of Pretoria. Somatic chromosomes were studied in root tips collected during late winter/early spring from vigorously growing potted plants. Actively elongating root tips ( 2 -5 mm) were collected at 14:00 and pretreated with freshly prepared colchicine (0,05% in distilled water) for 2h and fixed in Pienaar's (1955) fixative (methanol: chloroform: propionic acid in the ratio 6:3:2) for 16h. Root tips were hydrolysed for 12 minutes in a 1M hydrochloric acid solution kept at 60°C and stained in Feulgen for 2,5 h. Squash prepara tions were made using a standard technique (Van der Schijff & Robberste 1976). Photographs were taken with a Nikon FX 35A microscope fitted with a Nikon Optiphot camera. Measurements were made directly from mitotic preparations using an eyepiece micrometer. For chromo some nomenclature, the terminology introduced by Levan   Poellnitzia). For both genera the haploid set is asym metrical, producing a distinctly bimodal karyotype which consists of four long and three short chromosomes. No major chromosomal differences between Chortolirion and Poellnitzia could be detected. Furthermore, no marked size variation was encountered within the sets of long and short chromosomes in the respective genera.

DISCUSSION
The chromosomes of all plants studied were found to match the markedly bimodal karyotype which has been observed in every species of Alooideae to date. The basic number is X = 7 and comprises four long and three short chromosomes. To date a large number of intra-and inter generic hybrids produced in the Alooideae have been described and figured (Riley 1948;Jacobsen & Rowley 1955Rowley 1968Rowley . 1976Graf 1980 Levan et al. (1964). Chromosome II is clearly the LI (long chromosome with longest short arm) and chromosome I is the L2. which is often longer than the LI. Figure 3C). Satellites were detected at the ends of the short arms of the shortest long chromosome (IV) (Figure 3A, B)

. Secondary chromosome constrictions are absent from the long arms of the short chromosomes (V-VII). Such constrictions occur widespread in taxa of the Alooideae (Brandham 1971).
One chromosome I has a short arm much smaller than that of the other (Figure 3A, B). Since this phenomenon was not encountered consistently, it is probably not associated with an interchange or a deletion, but rather due to the orientation of the chromosome on the slide.  Brandham & Johnson 1977aCutler et al. 1980;Brandham 1982; Motohashi et al. 1985), none of the individuals included in the present study contained different levels of ploidy. In the Alooideae polyploidy can give rise to local bursts of speciation (Brandham & Cutler 1981;Cutler et al. 1980 Brandham & Cutler 1978. However, the identification of individual chromo somes (especially the short chromosomes) and their subsequent classification is very difficult and no attempt was made to do so here. Brandham (1983) eventually concluded that only LI (long chromosome with longest short arm) could be identified with certainty in somatic and meiotic cells. However, recognition even of the LI chromosome, which usually is the only submetacentric long chromosome, can occasionally yield difficulties as is shown in reports of Mogford & Rautenbach (1981), Vosa & Bayer (1981) and Vosa & Mogford (1981). In Chortolirion chromosome II is clearly the LI and chromosome I is the L2, which is often longer than the LI ( Figure 3C). In Poellnitzia chromosome I is the LI (Figure 4C).  (1972)]. In the present study no satellites were observed in Poellnitzia (Figure 4). The differences in karyotype morphology between published and observed data indicate the existence of chromosomal variability in Poellnitzia. It is noteworthy that considerable variation in the number and position of satellites has also been reported for other species of Alooideae genera (Brandham 1971;Spies & Hardy 1983). This karyotype character therefore appears to be taxonomically insignificant.

In the case of
Knowledge of chromosome morphology in closely related taxa is of primary importance in biosystematic and taxonomic studies. In the Alooideae studies of karyotypes are particularly informative, where the asymmetry in size within complements can assist in determining the progress of chromosome alteration and its consequences. However, some doubt still exists with regard to the phylogenetical significance of certain karyological aspects, such as overall size of chromosomes. Majumdar & Riley (1972) have, for example, suggested that in the species of the various genera of Alooideae, reduction in chromosome size is paralleled by a reduction in size and specialization of the plant. In contrast, Brandham (1983) has shown that, at least in Aloe. there is a gradation from smaller chromosomes in primi tive species to larger ones in advanced species. The latter are often highly adapted miniatures. Similarly, the chromosomes of Gasteria are larger than those of Aloe (Brandham 1990). Gasteria is generally regarded as phylogenetically more advanced than Aloe (Van Jaarsveld 1989). The karyotypes of species of Gasteria are also more acutely bimodal than those of species of the more primitive Aloe. Based on chromosome size and increased bimodality Poellnitzia would therefore appear to be more advanced than Chortolirion. However, the complexity of interpreting taxa of the Alooideae as either primitive or advanced, is illustrated by the suggestion of Hayashi (1987), based on callus characteristics, that Poellnitzia is a relict of early Alooideae. For Chortolirion, a close affinity was suggested with Haworthia subgenus Haworthia, a derived group. It is therefore clear that a multidisciplinary approach should be followed when drawing conclusions regarding the phylogenetic position of Alooideae taxa. In such studies wide-ranging karyological analyses will be of crucial importance (cf. Carter et al. 1984).