306 Cytologia 27 Chromosome Numbers in Oncidium Alliance1 Y. Sinoto Department of Biology, International Christian University, Mitaka, Tokyo, Japan Received July 10, 1962 The genus Oncidium of Orchidaceae includes more than 500 species, of which only about 39 species have been studied cytologically and counted their chromosome numbers (cf. Withner 1959). A typological research on the somatic chromosomes of the Oncidium alliance (cf. Dressler and Dodson 1960) has been conducted as one of the projects of the writer while staying in Honolulu as a visiting schalor of the East-West Center from October 1961 to July 1962. The results will pre liminarily be noted. Materials and methods Root tips were taken from the plants kept in several orchid breeder's nurseries in Honolulu. Those orchids in which chromosome numbers were determined are listed in Tables 1-4, which total 97 species and varieties, and 40 hybrids of 15 genera. Root tips were pretreated with 8 oxyquinoline (0.002M solution) to cause contraction of chromosomes for 4-5hours. Then they were fixed in modified Carnoy's fluid (1 chloroform+1 ethylalcohol+1 glacial acetic acid) for 5-30 minutes. The root materials were then hydrolyzed with 1:1 mixture of concentrated hydrochloric acid (12N) and 95% ethyl alcohol for 5-10minutes and stained with 1% aceto-orcein. All these treatments were made in room temperature of about 19•Ž. Obervational results For description of the results the plants studied are divided into four groups as follows: 1) Genus Oncidium. 2) Genera Erycina, Gomesa, Odontoglossum, Mil tonia, Brassia and•@ Sigmatostalix. 3) Genera Trichocentrum, Comparettia, I onopsis, Rodriguezia, Notylia, Macradenia and Aspasia, and 4) Hybrids. In 2) all genera of subtribe Oncidiinae (cf. Schlechter 1927) studied except Oncidium are included, while in 3) all genera of other subtribes (cf. Schlechter 1927) studied are included. 1 Contributions of the Department of Biology , Division of Natural Sciences, Inter national Christian University, No. 10. 1962 Chromosome Numbers in Oncidium Alliance 307 1) Genus Oncidium (Table 1) a) Chromosome numbers. Chromosome counts have been made for about 39 species of Oncidium by previous investigators . The numbers of chro mosomes were: 2n=10, 28, 34, 36, 38, 42, 44, 46, ca 48, 56 and 112. In the present work the following new 2n numbers could be added (Table 1): 2n=26, 30, 32, 37, 40, 84, 133 and 168. Besides these numbers , the Table 1. Chromosome numbers in genus Oncidium 1 Foster Botanical Garden, Honolulu. 2 Mr. Kirsch Orchidhouse, Honolulu. 3 Mr. Kirsch Orchidhouse. 4 Mr. Moir, Honolulu. 5 "Gigas" type, Mr. Moir. 6 Mr. Moir. following numbers were confirmed: 2n=10, 28, 34, 36, 38, 42, 44, 56 and 112 (Table 1). As Dodson (1957) pointed out, the unique interphase nuclei were also observed. In these nuclei "the chromatin materials form into large lumps and strands rather than a reticulum as in other species in the genus" (Dodson 1957). This, what may be called "papilio type", was also 308 Y. Sinoto Cytologia 27 confirmed in O. papilio, O. ampliatum, Odontoglossum grande, Od. schlieperi anum and Od. cariniferum. b) Karyotypes. There seems no report on the karyotype analysis in Oncidium. Generally speaking, the karyotype analysis seems difficult in Orchidaceae. A few examples of the analyzed karyotypes are shown below1: O. pusillum 2n=10. K=10(2b)=4Am+2Bsm+4Cst O. nanum 2n=26. K=26(2b)=2csAsm+2cBsm+2csCsm+ 2csDsm+2Em+2Fsm+2Gsm+4Hst+4cIst+4sJot O. harrisonianum 2n=42. K=42=12Acsm+18Bm+12Cm 2) Genera Erycina, Gomesa, Odontoglossum, Miltonia, Brassia and Sigmatostalix (Table 2). Table 2. Chromosome numbers in genera Erycina, Gomesa, Odontoglossum, Miltonia, Brassia and Sigmatostalix a) Chromosome numbers. The chromosomes were observed in one species each in Erycina, Gomesa, Odontoglossum and Sigmatostalix, and eleven species and varieties in Miltonia and six in Brassia. The numbers of chromosomes in these genera studied were 52, 56, 59, 60 and 86. In genera Erycina, Gonzesa and Sigmatostalix, the counting of chromosomes was first made in the present work. In Miltonia and Brassia the number 60 was found for the first time. b) Karyotypes. The karyotypes could not easily be analyzed also in these genera. All chromosomes in a set of Brassia chloroleuca seem to have similar shape and size, though all 60 chromosomes can be roughly divided into three groups, A, B, and C. Each of the A chromosomes has two constrictions, B's are median in their centromere positions, while C's have submedian centromeres. So the karyotype may be expressed as: K=60= 4csAst+24Bm+32Csm. Miltonia flavescens also has 60 chromosomes . Their shapes are almost 1 Symbols attached to chromosomes A , B, C,... are taken from Sinoto 1946. 1962 Chromosome Numbers in Oncidium Alliance 309 similar to each other except two A's, and they are continuously diminishing in size. So the karyotype may be shown as: K=60=2csAsm+4csBst+54Cm or Csm. 3) Genera Trichocentrum, Comparettia, Ionopsis , Rodriguezia, Notylia, Macradenia, Aspasia and Trichopilia (Table 3). The somatic chromosome number was counted as 28 in Trichocentrum albopurpureum by Dodson 1957, while it was 24 in the material in Honolulu . However, in T. panamense the number 28 reported by Dodson 1957 was confirmed. In T. maculatum and T. tigrinum the number was 24 . Cf. Table 3. Table 3. Chromosome numbers in genera Trichocentrum, Comparettia, Ionopsis , Rodriguezia, Notylia, Macradenia, Aspasia and Trichopilia The number 42 was counted in each of Comparettia falcata, C. speciosa, Notylia bicolor and N. panamensis, though Shimoya 1957 reported 32 in N. venusta. Blumenschein 1957 found 46 in Ionopsis paniculata and the same number was counted in I. utricularioides (Table 3). Macradenia brassavolae has 48 as its somatic number of chromosomes (Table 3), while M. paraensis has 52 according to Blumenschein 1957. Aspasia pusilla has 56 chromosomes, while A. principissa has 58 (Table 3). The number 56 is presumed in Tri chopilia suavis from its hybrid with another species whose chromosome number is known (cf. Table 4). Six species of Rodriguezia proved to have 42 somatic chromosomes each (Table 3). The chromosomes are rather small and uniform in size. 4) Hybrids (Table 4). The following 14 genus combinations were tested (Table 4): Oncidium •~ Oncidium, Oncidium•~Aspasia, Oncidium•~Brassia, Oncidium•~Miltonia, Oncidium•~Rodriguezia, Oncidium•~Trichocentrum, Brassia•~Aspasia, Mil tonia•~Miltonia, Miltonia•~Brassia, Miltonia•~Odontoglossum, Miltonia•~ Rodriguezia, Miltonia•~Trichopilia, Rodriguezia•~Rodriguezia and Rodri guezia•~Comparettia. 310 Y. Sinoto Cytologia 27 Table 4. Chromosome numbers in hybrids of Oncidium alliance 1962 Chromosome Numbers in Oncidium Alliance 311 a) Chromosome numbers. The chromosome numbers of most of the hybrids are the sum of those found in their parents (Table 4). For example, Oncidium Dr. Schragen is a hybrid between O. splendidum (36) and O. lanceanum (28) and has 32 (18+14) chromosomes. A hybrid between O. henekenii (40) and O. varicosum (112) has 76 (20+56) chromosomes.. Howe ever, there are some hybrids not having the sum of the parents' chromosomes (Table 4). For example, a hybrid of O. pulchellum (42)•~O. variegatum (40) has 61 chromosomes. It may be explained that an egg (21) was pollinated with a pollen grain having an unreduced number of chromosomes (40) resulting in the sum of 61. O. hispaniola is a hybrid between O. sylvestre (84) and O. flexuosum (56). Though it is expected to have 70 somatic chromosomes, it had only 56 which is the number of the male parent. In the case of a hybrid plant of O. haematochilum [0. lanceanum (28)•~O. luridum (32)] only 28 chromosomes were found which is the same number as that of the female parent. b) Karyotypes. It is generally difficult to discriminate the chomosomes of the parents. When there is a size or shape difference between the parents, it can be separated. Brassidium Supreme=O. papilio (38)•~Brassia maculata (60) is a good example. It has 19 larger chromosomes of the female parent and 30 smaller chromosomes of the male parent. General consideration and discussion A detailed consideration and discussion will be published in the full text. a) Basic numbers of chromosomes. In contrast with such groups as Dendrobium, Vanda, etc. which have rather a uniform chromosome number 2n=39, Oncidium showed so far a variety in chromosome numbers, being 2n=10, 28, 34, 36, 38, 42, 44, ca 48, 56 and 112. The basic number seemed to be at least 7. In the present work several new numbers have been added, i.e., 2n=26, 30, 32, 37, 40, 84, 133 and 168. From all these numbers, it may be presumed that the basic numbers b's are 5 and 7. In other genera, especially in Miltonia, Brassia and Rodriguezia, these two basic numbers 5 and 7 may also be expected. So b=5 series may be 10, 30, 40 and 60 and b=7 series 28, 42, 56, 84, 112, 133 and 168. Other hyper- or hypoploid numbers such as 32, 36, 37, etc. may be explained in any way as derivatives of the polyploid numbers of the basic numbers. The cause or origin of the polyploidy in these allied genera may be considered in several ways such as ecological, crossing, etc. b) Karyotypes. Karyotype analysis is generally difficult in the Oncidi um alliance as is in Orchidaceae in general. However, in some of the species studied such analysis could be made to a limited extent. Trabants, seta, etc. are seen in Trichocentrum and trabants or small heads are found in short arms of the two chromosomes of O. papilio and its Latour's variety. 312 Y. Sinoto Cytologia 27 In O. nanum the comparison of both size and shape among the chromosomes in a set could be clearly done.