Chromosome Numbers of the East African Giant Senecios and Giant Lobelias and Their Evolutionary Significancei

Chromosome Numbers of the East African Giant Senecios and Giant Lobelias and Their Evolutionary Significancei

American Journal of Botany 80(7): 847-853. 1993. CHROMOSOME NUMBERS OF THE EAST AFRICAN GIANT SENECIOS AND GIANT LOBELIAS AND THEIR EVOLUTIONARY SIGNIFICANCEI ERIC B. KNox2 AND ROBERT R, KOWAL Herbarium and Department of Biology, University of Michigan, Ann Arbor, Michigan 48109-1048; and Department of Botany, University of Wisconsin, Madison, Wisconsin 53706-1981 The gametophytic chromosome number for the giant senecios (Asteraceae, Senecioneae, Dendrosenecio) is n = 50, and for the giant lobelias (Lobeliaceae, Lobelia subgenus Tupa section Rhynchopetalumi it is n = 14. Previous sporophytic counts are generally verified, but earlier reports for the giant senecios of2n = 20 and ca. 80, the bases for claims ofintraspecific polyploidy, are unsubstantiated. The 14 new counts for the giant senecios and the ten new counts for the giant lobelias are the first garnetophytic records for these plants and include the first reports for six and four taxa, respectively, for the two groups. Only five of the II species of giant senecio and three of the 21 species of giant lobelia from eastern Africa remain uncounted. Although both groups are polyploid, the former presumably decaploid and the latter more certainly tetraploid, their adaptive radiations involved no further change in chromosome number. The cytological uniformity within each group, while providing circumstantial evidence ofmonophyly and simplifying interpretations ofcladistic analyses, provides neither positive nor negative support for a possible role of polyploidy in evolving the giant-rosette growth-form. Since their discovery last century, the giant senecios MATERIALS AND METHODS (Dendrosenecio; Nordenstam, 1978) and giant lobelias (Lobelia subgenus Tupa section Rhynchopetalum; Mab­ Excised anthers or very young flower buds of Lobelia berley, 1974b) of eastern Africa have attracted consid­ and immature heads of Dendrosenecio were fixed in the erable attention from taxonomists and evolutionary bi­ field in Carnoy's solution (3 chloroform: 2 absolute eth­ ologists (cf. Bentham, 1873; Hoffmann, 1894; Fries and anol: 1 glacial acetic acid, v/v). After 1 or more days the Fries, 1922a, b; Cotton, 1932, 1944; Bruce, 1934; Hau­ material was transferred to 70% ethanol for storage at 4 man, 1934, 1935; Humbert, 1935; Wimmer, 1953; Hed­ C. Anthers were dissected from buds in 1% acetocarmine. berg, 1957, 1969, 1973; Mabberley, 1973, 1974a,b, 1975; The anthers ofLobelia are sufficiently large that, for most Nordenstam, 1977, 1978; Thulin, 1984, 1985; Jeffrey, ofthe material, meiocytes were extruded from the trans­ 1986). They are premier examples of adaptive radiation versely cut anther sacs. Debris was removed, and the because of the large number of morphological features material (intact anthers or meiocyte tissue) was placed in and ecological syndromes that have evolved convergently a fresh drop of 1% acetocarmine, heated slightly, and in two families, the Asteraceae and the Lobeliaceae. One allowed to stain for 5 minutes. Intact anthers were washed problem is the identification of each group's closest rel­ three times with 45% acetic acid. A drop of Hoyer's atives and place of origin in eastern Africa. A second is mounting medium was added to the meiocyte tissue in the reconstruction of each group's evolutionary diversi­ 1% acetocarmine or to the intact anthers in 45% acetic fication. Mabberley (1973, 1974a, 1975), working within acid, and the material was squashed (Beeks, 1955). An­ the framework of Corner's (1949, 1954) Durian Theory, thers giving counts had lengths of 0.8-1.7 mm for Den­ considered the "pachycaul" (thick-stem) growth-form of drosenecio and 3-4 mm for Lobelia. Chromosome counts these plants to be ancestral and the herbaceous growth­ and photographs were made using a Zeiss phase-contrast form oftheir putative relatives to be derived. Most other microscope. Permanent slides are in the collection ofthe workers (notably Carlquist, 1962, 1980) have interpreted second author. the data as indicating an herbaceous ancestry with wood­ iness secondarily derived. Information on chromosome RESULTS AND DISCUSSION numbers, while giving only weak and partial evidence concerning origins and radiations, provides an essential The giant senecios and giant lobelias, although not aspect ofthe evolutionary history ofany group and allows closely related, are conceptually united by their conver­ other data to be more intelligently interpreted. gent giant-rosette growth-form and their parallel adap­ tations to a variety ofhabitats in the mountains ofeastern Africa. Parallels between the two groups also exist at the I Received for publication 28 September 1992; revision accepted 13 cytological level. January 1993. The authors thank the Tanzanian Commission for Science and Tech­ nology, the Ugandan National Research Council, the Institute Zairois The giant senecios-Most species ofgiant senecio were pour la Conservation de la Nature, the Office Rwandais du Tourisme discovered during the early colonial exploration ofeastern et des Pares Nationaux, and the Malawian General Secretary of the Africa, starting with Senecio johnstonii in 1884 (Mab­ Association pour l'Etude Taxonomique de la Flore D'Afrique Tropical berley, 1973). Initially placed with other arborescent se­ for permission to conduct research in their respective countries. This necios in Senecio section ArboreiO. Hoffmann (1894) and work was supported in part by NSF grants BSR-8800487 and BSR­ 8901123 and awards from the Rackham School of Graduate Studies, by itself in Muschler's (1909) redefined section Arborei, University of Michigan. the taxon was later raised to subgeneric rank by Hauman , Author for correspondence. (1935), a treatment formally validated by Hedberg (1957) 847 848 AMERICAN JOURNAL OF BOTANY [Vol. 80 as Dendrosenecio Hauman ex Hedberg. The giant senecios multivalents are common (Kowal, unpublished data). are inseparable from Senecio based on traditional char­ Meiotic regularity may be correlated with evolutionary acters ofthe heads and florets (Cotton, 1932, 1944; Hed­ age. The only irregularities seen are in one of the two berg, 1957; Mabberley, 1973, 1974a). However, using individuals of D. cheranganiensis ssp. dalei, where some their habit, pyramidal-paniculate inflorescence, more-or­ diakineses contain a univalent and/or one to three mul­ less biseriate involucre, and continuous stigmatic surfaces tivalents (Table I; Fig. 4). The woodland ssp. cheran­ ofthe disk styles in addition to minor differences in hairs ganiensis grows at the edge ofthe sedge meadows inhab­ and endothecial tissue, Nordenstam (1977, 1978) split the ited by ssp. dalei, and both flower at the same time, so group from Senecio (as with many other groups) to form hybridization is possible (Mabberley, 1971). the genus Dendrosenecio (Hauman ex Hedberg) Norden­ A gametophytic number of 50 is infrequent in the Se­ starn. Although initially unpersuaded (Jeffrey, 1979, 1986), necioneae, occurring sporadically in the Australian (Ar­ Jeffrey (1992) now concurs in light of recent evidence rhenechthites. Senecio), Ethiopian (Kleinia. Senecio), and (discussed below). Neotropical (Dendrophorbium, Graphistylis, Jacmaia, The large size of the giant senecios, while giving them Pentacalia, Senecio. Werneria) biogeographical regions a striking appearance in the field, often resulted in frag­ (Nordenstam, 1977; Jeffrey, 1979, 1992). However, so mentary and unrepresentative herbarium specimens far it has not been helpful in pointing out a likely sister (Knox, in press). This, along with the colonial subdivision group for Dendrosenecio. ofeastern Africa, contributed to an early proliferation of The radiation ofthe giant senecios may have been pred­ species descriptions. Hedberg's (1957) taxonomic revi­ icated on some genetic consequence of their high chro­ sion significantly clarified the circumscription oftaxa and mosome number, although this is not the case in the the applications ofthe numerous names. Mabberley (1973) radiation of the convergent Espeletia (Asteraceae, He­ was interested in evolutionary reconstruction but found liantheae) with n = 19 (Powell and Cuatrecasas, 1975; the "mosaic of variation" in the giant senecios difficult Hunziker et al., 1989). Certainly the constant n = 50 to interpret. He recognized two of the most distinctive indicates that their speciation involved no changes in species (one with two geographic subspecies) and sub­ chromosome number. merged the rest as infraspecific taxa of a polymorphic Identification of the closest relatives of Dendrosenecio Senecio (Dendrosenecio) johnstonii. Table 1 outlines a is necessary for understanding numerous evolutionary revised classification for Dendrosenecio (Knox, 1993). questions, including determining the ancestral chromo­ The ancestral (base) chromosome number for Astera­ some number and the timing between the origins of the ceae tribe Senecioneae, as well as Senecio itself, is x = 10 high number and the giant-rosette growth-form. Several (Ornduff et al., 1967). Unconvincing arguments for x = species have been suggested: Senecio subsessilis Olivo & 5 (Turner and Lewis, 1965) are further vitiated because Hiern (Engler, 1892), a coarse herb of upland eastern the n = 5 "Senecio" species on which the arguments are Africa with 2n = 40 (Hedberg and Hedberg, 1977), and based have been transferred to Emilia (Jeffrey, 1986). The other close allies in section Crociseris from Africa, such two most recent sporophytic counts (Hedberg and Hed­ as S. ochrocarpus Olivo & Hiern and S. karaguensis O.

View Full Text

Details

  • File Type
    pdf
  • Upload Time
    -
  • Content Languages
    English
  • Upload User
    Anonymous/Not logged-in
  • File Pages
    7 Page
  • File Size
    -

Download

Channel Download Status
Express Download Enable

Copyright

We respect the copyrights and intellectual property rights of all users. All uploaded documents are either original works of the uploader or authorized works of the rightful owners.

  • Not to be reproduced or distributed without explicit permission.
  • Not used for commercial purposes outside of approved use cases.
  • Not used to infringe on the rights of the original creators.
  • If you believe any content infringes your copyright, please contact us immediately.

Support

For help with questions, suggestions, or problems, please contact us