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The Indian Journal Genetics & Plant Breeding ( Published: October, 1968) THE INDIAN JOURNAL OF GENETICS & PLANT BREEDING VOL. 28 JULY 1968 No. 2 CYTOGENETICAL EVOLUTION OF CONIFERS. P. N. MEHRA Botany Department, Panjab Uniuersi[Y, Chandigark-14 I FEEL deeply grateful to the Executive Council of the Indian Society of Genetics and Plant Breeding for inviting metodeliver the Second Mendel Memorial Lecture. r have selected cytogenetical evolution of conifers as the topic for my lecture for two reasons. First, the conifers represent a compact group of plants that have had a long evolutionary history, having originated sometime during the Lower Carboniferous about 325 million years back and during this long span of their existence they have had ample opportunities of full expression of their evolutionary potentialities; and secondly, because conifers are of great impor­ tance to man from the forestry point of view and any exposition on their cyto­ genetical make up may help us in understanding the ways and means that could be usefully employed for breeding better varieties of trees. Conifers today are represented by 48 living genera and 480 species dis­ www.IndianJournals.com tributed throughout the globe and forming beautiful forests. In India, there Members Copy, Not for Commercial Sale are 14 genera and a total of 32 species met mostly in the great Himalayas ofDownloaded From IP - 61.247.228.217 on dated 27-Jun-2017 which II species are of considerable commercial importance. Although the conifers occupy barely 3·3 per cent. of the total forest area of the country, 25 per cent. of the commercial wood is obtained from them. Conservatively the conifers are divided into six families, the Pinaeeae, Taxodiaceae, Cupressaceae, Araucariaccae, Taxaceae and the Podocarpaceae. Somc persons, however, and justifiably too, havc segregated Sciadopitps of the family Taxodiaceae into a separate family Sciadopityaccae and Cepkalolaxus of the family Taxaceae into CephaLotaxaceae. We will now consider each of the above mentioned eight families separately. CYTOLOGY The cytology of conifers has attracted the attention of a number of workers but the chief contributions have come from Dark (1932). Sax and Sax (l933), · Second Mendel Memorial Lecture. 97 98 Indian Journal of Genetics & Plant B reeding [Vol. 28, No. 2 Flory (1936), Stebbins (1948), Mehra and Khoshoo (1956a, b), Hair and Beuzenberg (1958, Podocarps), SantamOqf (1960, Pines and Spruces), and Saylor (1964, 1966, Pines) . More recently Pederick (1957) has studied the chromosomes of Pinus Tadiata rather minutely. The chromosomes in conifers are generally long, and admirably suited for detailed study, particularly if the preparations are made from the haploid gametophytic tissue. T able 1 summari~es the present posicion in respect of the chromosome number and karyotypes of the members of this group worked out so far. TABLE 1 Chromosome number and karyotype in conifers Total no. No. of No. of No. of Name of genus of species Haploid acrocen· meta­ species worked number tries centries out PINACEAE Pinus 94 46 12 12 Picea 33 17 12 3 9 Ahies 45 8 12 5 7 Keleleeria 2 1 12 Tsuga II 3 12 3 9 Cedrus 4 3 I? 1 11 Larix II 8 12 6 6 Pseudot .... uga 6 1 13 7 6 Pseudoiarix 1 1 22 20 2 TAXODIACEAE www.IndianJournals.com Members Copy, Not for Commercial Sale Cryptomeria 1 1 II 11 Cunninghamia 2 2 J1 11 TaiwaniaDownloaded From IP - 61.247.228.217 on dated 27-Jun-2017 1 1 II Taxodium 3 2 II 1 10 Sequoiadendron 1 1 II 1 10 Mctarequoia 1 1 11 Athrotaxis 3 3 II Sequoia 1 1 33 Unworkcd genus: Clyptos/robus (1) SCIADOPITYACEAE Sciadopitys 1 10 CUPRESSACEAE Juniperus 48 15 (11 , 4) 11 ,22 10 Actinostrohus 2 1 11 11 Callitris 19 7 II 11 Tetraclim·s 1 1 11 II Widdringronia 5 1 11 11 Cupressus 12 11 11 10 ChamacC)"paris 6 3 11 Fil::roya 1 1 11 July 196B] Cytogmeticai evolution of conifers 99 TABLE l-Contd. Total no. No. of No. of No. of Name of genes of species Haploid acrocen~ meta- species worked numbt:r tries ccntrics out Thuja 5 3 II II Biota I I II I 10 Libocedr:ls 10 3 1\ Thujopsis I I II Unworked gcm:ra : Fokienia l2), Callitropsis (I) and Disclma (I) ARAUCARIACEAE Agathis 15 4 13 4 9 Arauearia 10 4 13 3 10 PODOCARPACEAE Podocarpus 64 31 10,11,12, 13,17,18, 19 Dacrydium 17 14 9,10,11, 12, 15 Acmopyle I I 10 10 Saxegotlwia I I 12 4 B PhylLodadus 5 3 9 9 Microcachrys I I 15 10 5 Pherospluura 2 2 13 6 7 CEPHALOTAXACEAE www.IndianJournals.com Cephalotaxus 5 2 12 I 11 Members Copy, Not for Commercial Sale TAXACEAE AmtnlotaxusDownloaded From IP - 61.247.228.217 on dated 27-Jun-2017 I I 1\ Taxus 9 5 12 2 10 Torreya 6 2 1\ Unworked genus: Auslrolaxus ( I) It will be observed that all the 9 genera of Pinaccac possess a haploid ch.om.o.s.ome number n = 12 with the exception of Pseudotruga which shows n = 13 and Pseudolarix n = 22. Larix is closely allied to these two genera but possesses ll= 12 chromosomes, six of which are metacentric and the remaining six are acrocentric. On the basis of the morphology of their chromosomes, it appears that the karyotype evolution in PseudoLsuga and Pseudolarix represents a case of fragmentation of onc metacentric chromosomes in the former or ten meta- and aero-centric chromosomes in the latter (Fig. 1) rather than a case of aneuploidy or polyploidy. In Taxodiaceae aU the genera are consistently characterised by the pos.<w.ssion of n = 11 with the exception of Sciadopilj's formerly included within it which has 100 Indian Journal of Centms & Plant Brteding [Vol. 28, No. 2 n = 10. This is in consonance with its morphological diversity from the other genera of the family necessitating its segregation into a separate family, Sciadopityaccae. Sequoia sC11Ipervirens, the monotypic representative of the genus, however, is a typical polyploid based on x. =- I1. A detailed analysis of its morphological features has led Stebbins (1948) to regard it as an auto-allohexa_ plaid involving perhaps a genome of Mdasequoia. Cupressaceae is allied to Taxodiaceae in many a morphological and embryological feature and this is reflected in its various genera also possessing the same base number X = 11. AIl the genera are at the diploid level with n=l1, the exceptions being four species of Juniperus which are tetraploid. PSEUDOTSUGA ~ ·1111 ii i ! ! ! II + ~ ~ It "; 'T' ~ ~ " ~ \ _';;" , \ \ \ \ , \ \ '--, • .\ ' \ \ \ .. ' J.. ; ..., \ I ,'\ \ \ 1_ --I ~ \ \ ~ I \ I \ \ J. - - \ \ • , \ \ \ \ .,..- ' \ \ \ • 1 ~ '. - '.,. - "I': ' \ .. " \ .. '. " ,.-~ I 1 I '" ... ... \, \ \ , I I I I , , \ www.IndianJournals.com I • I , , , Members Copy, Not for Commercial Sale , , , ,, , , " , ,, ,, , , ,, , , l'ARIX, ',. , , , , Downloaded From IP - 61.247.228.217 on dated 27-Jun-2017 , , , , I I I ~ , ,, , . , , , , , ,. PSEUDOLARIX Fig. 1. Karyotypic evolution of PUuMuU14 and PSndoldriJl from that of UriJt {after GustafUon and Mergen, 1964}. Both the genera of Araucariaceac, Araucaria and Aguathis, are inhabitants of the southern hemisphere and consistently show a haploid number of n = 13. All of the genera of Taxaceae hitherto investigated again show a haploid chromosome number of n _ 12. July 1968] Ct)'tQgeneticai evolution rif CQflifa s lOl • ..... ----~: . www.IndianJournals.com Members Copy, Not for Commercial Sale Downloaded From IP - 61.247.228.217 on dated 27-Jun-2017 -4:: , d a 3 FlO. 2. Somatic karyotype of Pi"", TO.>rburghij (after Mehra and Khoshoo, 1956). 1'10.3. Gametic karyotype of Abul /lWftS/li (after Mehra and Pathania, unpublished). 102 Inditln Jou 111 at 0if Gentlics & Plan'· B rud' mg [Vol. 28, No. 2 o www.IndianJournals.com Members Copy, Not for Commercial Sale Downloaded From IP - 61.247.228.217 on dated 27-Jun-2017 July 1968] Cytogenetical evolution of conifers 103 The position in Podocarpaceac, however, is very interesting. There is considerable variability in the haploid chromosome number but the basic plan underlying this apparent diversity has been beautifully brought out by Hair and Beuzenberg (1958). The total number of chromosome arms in the entire alliance is 20, which means that there are either 10 metacentric chromo­ somes or if the number is different, a metacentric chromosome is replaced by two acrocentric ones. In ACTTUJpyie, for example, there is a total of 10 chromosomes in the haploid phase, all of which are meta centric. In Saxegothaea, on the other hand, the number is n = 12 and here there are only 8 metacentrics and the 2 others are replaced by 4 acrocentrics. This state of affairs is visible not only in the different genera of the family but among the different species of the t\vo comprehensive genera Podocarpus and Daerydiu.m. The original basic karyotype in this family, which still retains many primitive embryological features, seems to be of 10 metacentric chromosomes which have exhibited a rather unusual tendency for fragmentation to produce variable numbers. AU the genera of conifers are characterised bya general constancy of their respective basic karyotypes with the exception of the two genera ofPodocarpaceae, Podocarpus and Dacrydium which show an unusual fcatwe of fragmentation of chromosomes in its various members as stated above. For example, in the genus Pinus all the species investigated show 12 pairs of metacentric chromosomes with the smallest pair possessing a distinct sub-median constriction (Fig. 2). In the section Larciones, however, a second distinctly sub-median chromosome has been shown to exist by Saylor (1964). While the basic karyotype remains cons­ tant, there are, however, minor variations in respect oftbe number and position of secondary constrictions noticeable in the different species of the genus. In Abies there are 7 metacentric chromosomes and 5 acrocentrics.
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