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Meiosis in Polyploids the Number of Long Chromosomes with a Second Constriction (Darlington, 1926) Was Identified It Is Denoted by the Letter L I

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Meiosis in Polyploids the Number of Long Chromosomes with a Second Constriction (Darlington, 1926) Was Identified It Is Denoted by the Letter L I C. ]). ])AlZLINGTON 17 PART II. ANEUPLOID HYACINTI~tSL BY C. D. DARLINGTON. CONTENTS. PAC:E 1. ] N'I:1l O1"~ UO'I'I 0I,~ 17 2. Mz'nmu8 18 3. M,vrsRIm~ . 19 4. SOMATI0 :DIviSiONS 19 5. POLLNN ~{O'I'HER-tJELL DIVISIONS IN TIIlSOMI0 VARIETIES 20 (i) Oeac.'~l 20 (ii) Nobe on the Diploid . 22 (iii) First MeUq~h~o 22 (iv) Anaphasc: Division of the Trivalo.t. 28 (v) Second J)ivislon 31 (J. I:~OLLEN 1%IOT]INIt-OBLL DIVISIONS IN T:I~TIIASOMIO VARIETIES 32 (i) Tho Tetrasomio Variotios . 32 (ii) First MOtal~hase 32 (iii) Second Division 37 7. l~ItOl'IthSE O~" 'mE POLLEN ~'[O'I'[LER-OI~SLDiViSIONS 38 (i) Triple ]?airing 38 (ii) Quadruple Pairing 39 8. DlsoussloN . 41 (i) Mult{p|c Association of Cl}l'OnlO8OlllO8 .. 41 (ii) lndlrc~;l~ .Evidence on the Origi. of Chlasmat~, 42 (iii) Gc.cl~ical Considerations . 43 (iv) Chromosome Matiug and Crossing Over 47 9. St~tMARY ,52 RE FERENCEH 54 EXPLANATION OF PLATES 56 NOTE ON 'HIE DIAGRA~I 56 I. INTRODUC"rION. DE MOL'S work revealed in the hyacinths maberial of pecldiar in- terest for fm'ther cytological study. It was evident that individuals having almost every possible chromosome combination between diploid and triploid and between triploid and tetraploid had given rise to the ctflMvated clones of hyacinths. Moreover the chromosomes were of three sharply differentiated types having, as was shown later (Belling, 1925; Darlington, 1926), definite and consbant constrictions. Finally, being derived, so far as we know, ~4thout any ga'oss hybl~disation from a diploid species, the homologous chromosomes might be expected to be x I,mludl.g triploids. Joum. of Oon. xxI 2 18 Meiosis in Poly2~loids indi'fferently attracted to one another in whatever numbers they occurred. Thus the hyacinths ofered an opportunity for the comparative study in the same cell of the structure and behaviour of bivalent, trivMent and quach'ivalent chroln.osomes of distinct types. Belling has already described the metaphase of the first pollen mother- cell division from aeeto-earmine preparations in both diploids and tri- ploids. At the present moment I am not directly concerned with the diploids, my purpose being, first, to show the relationslfip between com- pound structures at metaphase and their antecedents at prophase, and, secondly, to describe the reduction of these structltres and the effect it may have on the general com'se of division. The difference in method and in point of view makes these studies complementary to Bellfllg's. 2. METtlODS. The methods were similar to those used with the tulips, but observa- tions of early prophase were made fl'om sections as well as from smears. Hyacinths are less well suited for smearing than tlflips, ~specially in prophase of the pollen mother-cell division, on accmmt of the small size of the anthers. The prophase nucleus is apparently less resistant to the action of fixatives than is the case with the tulips, and a period occm's between the completio n of pairing at pachytene and diakinesis when fixation-- judging from the irregtdarity in the thiclmess of the chromosomes, par- ticularly theft' g~'eater contraction at flee ends than when attached to nucleoli, their less even distribution and more angldar outline is usually less satisfactory than in the stages immediately preceding and following in the same preparations. At this stage the nuclei are evidently more seusitive to the action of fixatives. This sensitiveness occasionally takes the form of contraction to one side of the cell, and might be regarded as synizesis. For this reason the post-diplotene stages will not be described. A similar condition is sometimes fmmd in the tulips although by no means so pronmmeed, and it appears that fixation by our present methods cannot be comparable at all stages of prophase. Bgla~ has advanced the view that two processes go on side by side at prophase of division; first, a reduction of viscosity in the "Kern- grundsubstanz," secondly an increase in the viscosity of the chromosome owing perhaps to loss of water to the "sap." A point may then be reached when the viscosity of the supporting medium has been reduced without the rigidity of the chromosomes being sufficiently increased to enable them to stand the el~ects of treatment, especially with fixatives. In C. D. DARLINGTON 19 these circumstances more Of less pl'ommneed contraction will follow fixation. This theory aeeom]ts fox' the behaviom' of the prophase nucleus in Hyacinlhus, and acemmt,s also fox' the still greater difficulty of studying the critical stooges of pairing in most plants. 3. IVIATEIII A L. The probable origin of the triploid hyacinths has been referred to in the first part of this study. They are remarkable, probably exceptional, in one other way; they are highly fertile. As De Mol has shown--and his result,s have been confirmed with several varieties--they are self-sterile, but when crossed give a highproportion of good seed. This fertility extends even to the approximately tetraploid clones Totnla (2n = 30) and La Grandesse (2n = 28). A whole spike of Totnla selfed gave one seed; the flowel~ of another spike crossed with La Grandesse gave 160 seeds. Although self-sterility is absolute in the triploids, De Mol found, as here in Totula, that exceptional selfed seeds may occm: in Ax'entine Arendsen, which I find also has more than the triploid nmnber (2n = 28). Perhaps correlated with the ordinax T fexgility of these aneuploid hyacinths is the fact that hyacinths appear to be equally successful with every chromosome number between 16 and 30, so that gametes with vcry irregular chromosome numbers are no less likely to produce viable zygotes. The question as to whether IIyac6~thus o~ienlalis with the somatic mlmbcr of 16 can in any way be considered a tetraploid is of importance in relation to thane studies. My earlier opinion (1926) that the presence of a single long chromosome with a second constriction prevented our regarding the species as haxdng been derived by simple doubhng has been confirmed by the stndy of the pollen mother-cells. There was strong evidence of tetraploidy in one preparation of prophase in Grand Maitre which I attribute t~ the bulb being a misnamed tetrasomic form. One or two similar suggestions at metaphase in other triploid varieties I have been xmable to confirm. There is little doubt therefore that the species is a flmctional diploid. L~. SOM:ATIODIVISIONS. As a preliminary to studying their pollen mother-cell divisions, root- tips Of a number of varieties, some of which lull been described by De Mol, were examined. The following table summarises the results, L, 11I and S being the long, medium and short chromosomes respectively. Where 2-2 20 Meiosis in Polyploids the number of long chromosomes with a second constriction (Darlington, 1926) was identified it is denoted by the letter L i. Nimrod (Texbfig. 19, 2~ + 3) ... 11 L (3 L i) 4. M 4. S City of Haarlem (3n - 1) ... 12 L 6 M 5 S Marconi (3n) ......... 12 L 6 M 6 B La Peyrouse (3n + 2) ...... 14. L 6 M 6 g Arcntine Arendsen (3n + 4.) ... 14. L (4. L i) 6 M 8 S The pollen mother-cells of the variety Totula, in whi.eh somatic variations in number had been found, disagreed with both De Mol's results and my own in regard to the proportions of the different types, having the constitution 15 L, 8 M, 7 N. The counts of Nimrod and City of Haarlem also disagree with De l~{ol's results, but are confirmed from the pollen mother-cell divisions. 5. rOLLEN i~{OTIIER-CELLDIVISIONS IN TRISOMIC VARIETIES. (i) General. At metaphase in diploids, and less regularly in aneuploids, the long chromosomes lie flattened in the plane of the equatorial plate and appear very little contracted as compared with somatic di~dsions. At anaphase each of the bodies passing to the pole is seen to be double, the two halves (which are to pass to opposite poles at the second division) being already entirely or almost entirely separated. This will be dismissed in the light of special observations later. At interphase the daughter-nuclei pass into a resting stage, and a cell wall divides the pollen mother-cell before the metaphase of the second division. At this point the chromosomes are only held together at the attachment constriction. It is a remarkable fact that although compound bodies are formed with great regularity at metaphase in the polyploid hyacinths there is yet no trace of any other kind of attraction between homologous chro- mosomes, whether paired or unpaired, before or after division. Chro- mosomes having once separated from one another in the division of a trivalent, at the first metaphase, show no affinity for one another at the second; nor does an odd chromosome, which has failed to form a ~rivalent with a pair homologous with it at the first metaphase, show any attraction towards this pair. Thus, although the division of the trivalents in the hyacinths is comparable, for example, with that in Prunus (Darlington, 1928), their later behaviour is distinct in the entire absence of secondary pairing. This question will be discussed later. Text-fig. 19. Somatic metaphase of the hyacinth variety Nimrod with 11 long chromosomes, 3 having ~ second ooush'ic~iou, 4 medium cliromosomesand 4: shor~, x 2~LO0. Text-fig. 20 ev-d. l~o,,r first metaphases in tile hyacinth va.riety Roi des Beiges. y. 2400. Text-fig. 21. Diakiuesis chromosomes of the h.il)lo]d variety Lady Derby; all trlva.leng oxeel)t one short bivalent a~ld univalent.
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