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Sex Chromosomes in Macromitrium

1966; American Bryological and Lichenological Society; Volume: 69; Issue: 3 Linguagem: Inglês

10.1639/0007-2745(1966)69[293

1938-4378

Helen P. Ramsay,

Marine and coastal plant biology

Chromosome numbers of n = 8, 9, and 11 have been found in the five species of Macromitrium examined. Evidence is presented in support of an X/Y sex chromosome mechanism in which the X chromosome is larger than the Y. These morphologically distinct chromosomes occur in dioecious species having chromosome numbers of n = 8 and n = 9 and form a dimolphic bivalent at first metaphase of meiosis. In monoecious species, however, a chromosome number of n = 11 was obtained with no dimorphic bivalent. The behavior of the sex chromosomes was observed at meiosis and the X could readily be traced to half the resulting spores, the Y to the other half. Correlated with the chromosome differences was an heteropycnotic difference visible at telophase of the first and second meiotic divisions. Dioecious species were found to be heterosporous, and the range of size in spores of species of Macromitrium with n = 8 chromosomes gave a bimodal distribution. This heterospory is further related to chromosomal and heteropycnotic differences. In monoecious species all spores contained identical chromosome complements and heteropycnotic characteristics. There is some evidence to indicate that such species contain both the X and Y chromosomes. Macromitrium Brid. in the Orthotrichaceae includes over 400 species. The genus is almost entirely corticolous, but a few species occur also on rocks. The branching secondary protonemata produce many upright branches which in turn bear the archegonia and/or antheridia. The plants become massed to form dense mats along the trunks of trees and on fallen logs in the rain forests of the tropical and subtropical regions (Herzog 1926, Sainsbury 1955). Monoecious and dioecious species occur, and in many of the dioecious species the male plant has been reduced to a tiny epiphyte on the leaves of the female (Brotherus 1909, 1924). The spores of dioecious species having reduced males are often heterosporous (Ernst-Swarzenbach 1939, 1942, 1943, 1944). The presence of sex chromosomes in Macromitrium has long been suspected because of this apparent heterospory. Ernst-Swarzenbach showed that in 28 dioecious species studied, 20 had spores of two sizes. Germination experiments led her to conclude, contrary to Fleischer (1920), that the larger spores produced female plants while 1 I am indebted to Professor G. K. Berrie who suggested the problem and encouraged me in the investigation, to Professor S. Smith-White for his criticism of the manuscript, and to Mr. J. H. Willis who kindly assisted in the identification of the species. 2The School of Biological Sciences, University of Sydney, Sydney, New South Wales, Australia. This content downloaded from 207.46.13.153 on Fri, 05 Aug 2016 05:03:53 UTC All use subject to http://about.jstor.org/terms 294 THE BRYOLOGIST [Volume 69 the smaller gave rise to the tiny epiphytic males. She examined altogether 35 species from many parts of the world including homosporous and heterosporous species, and variation in spore size was shown graphically. The homosporous species gave a very narrow normal distribution, while the heterosporous species gave distributions with two widely separate modes (e.g., M. salakanum), or two modes close together (e.g., M. blumei). She did not consider these latter to be truly dioecious species. Recently Miyoshi (1963) investigated spore size in five Japanese species of Macromitrium including one previously investigated by Ernst-Swarzenbach. His results agreed with those of Ernst-Swarzenbach, showing a single mode for the distribution in homosporous species and two modes as in M. blumei for the heterosporous species. The necessity for cytological studies to supplement this work and to determine whether distinguishable sex chromosomes are present in these heterosporous and dioecious species has been stressed (Allen 1945; Steere, Anderson & Bryan 1954; Wylie 1957; Lewis 1961). The present study includes, as well as a report of chromosome numbers for five species representing 14 populations of Macromitrium, a comparison of the spore size of monoecious and dioecious species and an investigation of chromosome size, behavior, and heteropycnotic characteristics of these species. TABLE 1. Summary of the chromosome numbers in species of Macromitrium. Collections were made at Happy Valley (Ia), the waterfall (Ib), Cathedral of Ferns (Ic), and Zircon Creek (Id) of Mt. Wilson (3,200 ft); Megalong Valley (3,000 ft) (II); and Oxford Falls (500 ft) (III). The species are indicated as monoecious (M) or dioecious (D) and the spores as being of similar (S) size or variable (V) size. Voucher Sex Spores Locality n Figures M. (EUMACROMITRIUM LEIOSTOMA) 41/64 M. daemelii D V Ia 8 1-10, 22-32 M. (GONIOSTOMA) 40/63 M. archeri D V III 8 33-34 32/65 M. cfr. ligulare D V II 9 19-20, 45-47 27/63 M. cfr. ligulare D V Id 9 11-18, 37-44 29/65 M. archeri D V Ic 9 55-58, 62-64 30/65 M. archeri D V II 9 35-36 35/65 M. archeri D V II 9 28/65 M. cfr. scottiae M V Ib 11 21, 48-51 41/65 M. cfr. scottiae M S Ib 11 52, 59 M. (MACROCOMA) 28/64 M. eucalyptorum M S Ib 11 53, 60-61 25/65 M. eucalyptorum M S Ib 11 34 / 65 M. eucalyptorum M S Ib 11 37/65 M. eucalyptorum M S Ib 11 54 39/65 M. eucalyptorum M S Id 11 This content downloaded from 207.46.13.153 on Fri, 05 Aug 2016 05:03:53 UTC All use subject to http://about.jstor.org/terms 1966] RAMSAY: SEX CHROMOSOMES IN MACROMITRIUM 295 TABLE 2. The percentage of spores of various sizes in the species of Macromitrium studied. Voucher Spore size (L,) Voucher 8 12 16 20 24 28 32 36 40 8 12 16 20 24 28 32 36 40 28/65 M. scottiae 0.6 6.5 22.5 42.4 24.6 3.3 29/65 M. archeri 5.3 45.5 35.1 10.0 3.7 0.3 27/63 M. ligulare 1.0 6.4 49.6 30.3 7.2 3.4 1.9 41/64 M. daemelii 5.7 23.1 39.0 10.4 11.7 8.4 1.0 0.8 40/63 M. archeri 0.5 13.4 31.4 21.9 24.3 7.7 0.5 MATERIALS AND METHODS The populations of Macromitrium investigated were collected from the Blue Mountains district of New South Wales except for M. archeri (40/63) which was obtained from Oxford Falls near Sydney by Miss H. Hewson. The specimens were kept in the laboratory at room temperature (15-21?C). Smear preparations were obtained in the usual way (Ramsay 1964), stained in aceto-orcein, made permanent by a carbon-dioxide freezing technique, and mounted in Euparal. Mitotic counts were obtained from growing tips using Lowry's (1948) method, and the slides made permanent as above. Slides for spore-size data were made by releasing spores from mature capsules into fixative, then mounting in aceto-orcein. Measurements were taken using an ocular micrometer before the slides were made permanent. All the spores on each slide were measured. Drawings were made with the Abbe camera lucida using a Zeiss X 12.5 ocular and a X 100 oil-immersion objective. Initial magnification was X 2,700 and all drawings (except those in Fig. 55-58) are reproduced here at X 1,350. Photographs were obtained under the technical direction of Mr. R. Oldfield using Zeiss equipment and phase-contrast microscopy. They were developed and printed with assistance from Mr. B. Lester and are reproduced at a magnification of X 1,100. Mr. J. H. Willis has kindly examined the specimens and given suggestions on the identifications. He points out that Macromitrium is in need of revision, as the limits of Australian species are not well understood. The identifications are, therefore, given in this paper within the limits of present knowledge. Where no definite determination was possible, affinities are indicated and a tentative name used. The voucher specimens with comments are deposited in the Ray Herbarium, Botany Department, University of Sydney (Table 1).