Variation in first generation hybrids (imperfect dominance): Its possible explanation through zygotaxis

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\ Y. T. S\ YIXGLE. - YAlUATiOX L'; FIRST GENERATiON HYBRIDS. 581
VARIATION IN FIRST GENERATION HYBRIDS ( IMPERFECT DOMINANCE)
ITS POSSIBLE EXPLANATION THROUGH ZYGOTAXISl
By Walter T. SWINGLE
U. S. IJcpa" lmelll or agrieullure, Washington, IJ. C.
Current theories of herediLy and variaLion give no meLhod of explaining
variations in firsL generaLion hybrids between pure bred parents.
It is genemlly assumed LhaL differences ill Lhe characLers of organisms arc
due Lo corresponding differences in the eomposiLion of Lhe gameLes Lhal uniLe
to form the individual organism. No doubL LhaL is tme in many cases, but iL is
my purpose Lo point ouL cases in , yhich Lhis assumpLion cannot be susLained.
Some fi rst generaLion hybrids show a very considerable range of varia Lion ,
far more than can possibly be accounLed for by any qualitative differences in Lhe
chromosomes or oLher bearers of herediLy received from the Lwo parenLs.
Inasmuch as during Lhe "' hole of the first ( or conjugate) generaLion Lhe
chromosomes from Lhe t\ Yo gameLes persist side by side in an unfused staLe,
such variations cannoL be due Lo any quanLitaLive differences in Lhe bearers of
herediLy possessed by the sistel' hybrids.
If proof can be given Lo show Lhat in cerLain specific cascs pairs of gametes
of identical hereditary composiLion give rise to very diverse organisms, the way
has been opened for a general reinvestigaLion of the validity of Oul' modern
Lheories of heredily.
Some years ago "' h\:, n I carried on breeding ,,' ork on maize in conjunction
wiLh my teachel', the laLe Prof. ' V. A. Kellerman, I was pl'Ofoundly impressed
by Lhe diversity ShO\ YIl in different planLs of first generation hybrids of idenLical
pure parenLage. Alllhe cars on a single plant, no mailer how many, were of
Lhe same general Lype, but those on other planLs descended from the same L,,' o
races were different, often very decidedly different. A considerable number of
hybrids showed this behavior!.
It must be remembered that such hybrids are made up of cells, Lhe nuclei
of which contain side by side Lhe identical chromosomes derived from the Lwo
parenLal gameLes. If Lhe parenLs are pure bred, all the first generation hybrids
w'e derived ( rom identical gametes cmd have an ident~ cal equipment o( chromo­somes".
Any difrercnccs obscrved beLwecn such hybrids must be due Lo difrerences
in the expr'ession of herediLary tendencies and noL due to differences in their
transmission Lo Lhe gameLes as Cook has poinLed ouL '.
In recent years I have found striking dillerences beLween first generaLion
sisLer hybrids of two vcry difrel'ent species of Citl'U: 3, which aILhough showing
1. Communication faile iJ la lroisieme seance de la Conrerence.
2. Kellcrman, W..\., and Swingle, ' Yo T., 1~ 90. Crossed Corn the Sec. Jnd Year, in 2nd .\ nnual
Heporl Kan. Agl'ic. Exp. Sta. pp. ;): 54-: 546, pI. 11. Topeka. •
: 5. Cook, O. F., amI Swingle, W. T., 1905. Evolulion of Cellular Struclures. Bull. 81, B. P. I., U. S.
Dept. Agricullw- e. ( Aug. 4).
4. Cook, O. F., l( l07. Transmis: 3ion Inheritance distinct fr01l1 Expression Inherilance in Science
(:'\. S. 25: 911- 912 n. 649, 7 June).
58~ lY' CO~ F£ IH~ NCE J: iTER~ ATlONALE DE G8NI~ TJQUE. ,.
some slight variations, arc relati vely pme bred in contl'ast with the wide chasm
separating the very ditTerent species c!' Ossed to obtain the hybrid.
In J897 I crossed the common sweet orange ( Cill'US AuranliUlll, sinellsis L.)
with the Oriental trifoliate orange ( Citrus ll'i( aliatc6 L.), a deciduous species very
distinct from the common cit!' Ous fruits L.
" In comparison \\' ith the "' ide chasm that separates these species, the varia­tions
they show inside their respective limits arc insignificant. In other ,,' ords,
for purposes of genetic study, the germ cells may be considered pure in so far
as n~ ither of the pal'Cnt species show variations that to any perceptible degree
Fig. l. - Colmln cill'ang~, s! IO'.,' ing thc spots due to fUl130~ 1S grJWlll amo: lg tlLc fllzzy hairs cUl'ering
thc skin. Thc uil glands arc " Cl'y small. Reduccd I/ i.
briJg\~ the ,,, ide gul(, separating them. In faeL, the olJer kees 0(' Cil?' uS lri( a­Ziata
g!' Owing in America arc all descended from a very few plants imported
from Japan some furty years ago ami show few variations and these 0(' minor
extent 2 •
The sweet orange was grown f!' Om seed (' 01' a couple of centuries in Florida
before the Emopean varieties were introduced by General Sanford, the Rev.
Lyman Phelps, and other progressive men about half a century ago. It is tme
that a number 0(' vaeieties of so- called Florida oranges were found among these
tens of thousands of seedlings, but they arc all of the same general type and
cannot for a moment be held to obscme the p!' Ofound dilTerences sep31' ating,
them, and in faeL all known sorts of the common orange, f!' Om Cit,' us lri( aliala.
Nevertheless, eleven of these sister hybrids g!' Own in 1897 from seeds of a single
I. Thc Oowcr buds a1' C fur IIIcd cal'll' ill sumlllcr and opcn thc fullowing sprill'; 011 olci 1I' 0ud, as in
thc ca, c of mall)' olhcr dcciduous trces, TILc slamcns are frce, thc pislil i- 8 celled, the pulp I'Csicics
bal'e glan tular sccrcting hairs 011 theil' surfacc, the pith has trans,' crse plates of thi~ k-\ I'alled cells, the
fruits are downy hairy, thc petals are c1aw~ d, thc leavcs trifJliate, thc lhl\\' CrS arc ncal'ly scssile. In
all of thesc particulal's. C, trifoLiat( l diJl'er' from an~' othcr spccics of Citrus.
2. Thc chicf Yal'ialion is a tcndency to staminody uf the pctals, lIlakillg cCI, t, Jill plants small­]
Ioll'crcd 01' cl'cn apetalous.
W. F. SWINGLE. - VAlUATION IN FIRST G'I~ NERATLO~ UYBRlDS. 383
fruiL of Cill'US l1' i( oliata crossed with pollen of a single flower of the common
orange showed considerable diversity in leaf characLer and very striking diver­sily
in Lhe fruit characLers. One variety of cilrange as Lhese new hybriJes arc
called Lhe « Morlon )) has very lat'ge, round, smooth, orange- colmed fruiLs j
anoLher, Lhe « Colman)) ( Fig. ' 1), has depressed, globose, yellow, fuzzy- hairy
fmiLs; the « vVillits )) ( Fig. 2) has a large pCl'centage of fingered fruits j the
Fig. 2. - Willits Cil. l'ange, showing a fingered fmil. common in tlJis rariely. One fruit [ rom abore,
one from the side ( sectioned). Heduced ' 1/ 4.
« Hustic )) ( Fig. 5) often has double fruiLs, a smaller fruit siLuated on lop of
the larger one, and lhe calyx is frequently enlarged. The « Phelps)) is very
biLter, \" hile the « Saunders)) ( Fig. 4) has small fruits with scarcely a Lrace of
bitLer in the juice. Everyone of these eleven hybrids is decidedly different
fWIll all the others in its fruit characters and some, like the depressed, fuzzy
« Colman)) and the fing,' red « vVillits )) are slrikingly different I.
1. Thes~ citt'ang- es have been described in the following papers:
Swingle, Wullel' ' 1'., and Webber, Ilerbert J.. ' 1808, - Hybrids and Tlieir Uliliwlion in Plant
lit'eeding, in letTbook Dept. Agrk. for 1807, p. 415, fig. 13.
Webb2r, Hcrbcrt J., 1000, - Work of thc United Stutcs Departmcnt or Aglicullurc on Plunt lIyLl'i­dizution,
in Journal Iloyul Hort. Soc" Loudon, 24: 128- 138, 144, figs. 42-\ 7. Also rcprintcd scparately.
pp. 1- 11, n, figs. ' 1- 6.
WeLbcr. Herbert J., and Swing- lc, WallcrT.. 1005, ;' icwCilrusCreutions of Ihe DepuJ~ nent o[ Agri­culture,
in Yearbook Dept. Agric. [ or ' 1904, pp. 21' 1- 235, figs. 12- 13, 1' 15. 10- 16.
Webbcr, Herbert J., 1006, - [' iC\\' Fruit Productions or 1lie Departmcnt of AgTiculture, in YeurLoQk
Dcpt AgTic. [ or 1003, pp. 273- 278, lig. 80, pIs. ' 17- 10.
WebLcr, HerL" rt J., HI1l7, - ~ ew Cill'US rind Pincapplc Productions or the Dcpartment o[ Agricul­.
ture, in Yeal'book Dept. Agric. [ or 1006, pp. 529 · 336, iig. 10, pIs. 17- 20.
IV' CO, WEHE~ CE L\ TEHNATlOXALE DE GE.\ ETIQUE.
These citranges are a ne\\- type of acid citrous fruits decidedly more resis­tant
to cold than any now grown. They can be used for making acid drinks
and for culina!'} purposes in place of lemons or limes. They are being grown
Fif(- 3. - lIustic citl'augc, iH cro, s scction and side ricw showiug a douLlc fruit, Hot infrequent with
this variety_ Rec! Llced J/~.
in very many parts of the southern United States whcl'c no orang'es or lemons
could stand the severc winter weather.
Great as are these diffcrcnces in first generation hybrids bet\ yeen Lhe orangc
Fig. 1. - Saundcrs citrange, showiug L1lc skin rouglJcllcd Ly L1lC ycry largc aud promillcllt oil glallus.
I\ cuuced ' 1/ 4.
and Citrus tri( oliala, still greater differences have comc to light in those bet­ween
the lemon and Cilrus tri( oliata. All hybrids between the sweet orange
and Citl'US tl'i( oliala ( citranges as thcy are callcd) haye trifoliate leaves and
germinate in a normal manncr. Hundreds of new citranges have been produ­ced
within thc last few year~, so it is possible to speak wiLh assurance on this point.
\\". F. SWINGLE. VARIATJON L'\ FIHST GE~ RHAT[ QN ! iYBRIDS. 51\ 5
Fig. 3. - Cill'emOIl. ;\". 46730 ( Lishon lemoll crossed willi Citrus tri{ oliata). Leaves mosLly in form
of bypophylls. Cill'emons sllo\\' neal'll' 20 per 100 of seedlings \ I'ilh an exaggerated deYelopmrnt
of hypophylls. Nalul'al size.
I\. e f: O: XFER. I~ Tf. n:\ · . UE G..~.' q.~ TIQUE. C)~
_.)
IV" CO:\ FI~ nENCE I~ TEBNATIONALE DE GENlhtQUE.
J
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A" 4 ~
... ..- -~-~._.~...,... ~
Fi~ · . G. - Cill'lIS Il'i( olirtlrt, 1\ 0 09) IT4 · 01. A seedlillg plant,
showillg Ihc hypop" yll,; alollg Ihc lowcr pm't of the slem.
This spccies crossed with the orangc produces thc cilrang'c;
crosscd willi the Icn, on, thc citrclllon. l'iatu,' al sizc.
The citl'emons, or crosses between the lemon and Cil1" us tTi( oliata, on the
contrary, show nearly 20 per cent of seedlings with an exaggerated development
of hypophylls and in the majOl'ity of cases never produce any normal foliage
leaves at all ( Fig. 5) dying from starvation shorLly aner the reserve food mate­rial
of the seed is used up. One parent, Citnts l1' i( oliata, has a few hypophylls
along the base of tbe stem of the young seedling ( Fig. 6), while the lemon, like
the orange and all other
common ci trous fruits,
sho, ys a pair of rather
large, rounded, sessile,
opposite leaves as the first
post- cotyledonary foliage.
Tbe excessive developmen t
of hypophylls represen ts
an intensification of one of
the parental characters.
Bcsides the seedlings
with hypophyIls, many of
the citremons show wide
variations in the character
of tbe leaves. Some show
five leaflels ( this occasio­nally
occms in citranges
also)', others show u ni fo­liale
leaves 01' leaves with
very much reduced side
Jeaflels ( Fig. 8) ( which is
rarely or never seen in ci­tranges).
" lost of the ci­trcmons
have trifoliate
leaves with large laleral
leaflets ( Fig. 9), which is
the type characteristic of
cilranges.
There arc several hun­dred
of these hybrids under
stuJy. Three yarieties of lemon, the Eureka, Villa Franca and Lisbon ( Fig. 10),
were used in cros~ ing with Cit1'/ l8 tl'i( oliata and all three yielJed a considerable
proportion of seedlings ( usually about one- fifth) with pronounced hypophylls.
As yet none of the citremons have borne fruit, so it is not possible 10 compare
the variations in the fruits with those of th'e leaves. Doubtless the striking fmit
variations shown by the citranges will be equaled or more probably exceeded by
the citremons, since the latter show a dccidedly greater range of foliar variation 2.
' 1. Somc or thc cilranges obtained by crossing thc ' Yashinglon I\ avel orange wilh Cilnts Il'ifuliala
show a considerablc proportioll of 5- foliatc leaycs ( Fi!!". 7)- a cha'aclel' ncvcr cxh · bited by Cilnts
l1 · i/' oliala.
2, As a malle" of fael, Ihc thrce hybrids in Figs. 5,8 and 9 al'C not sisler hybrids, grown from
sccds of a single fl'Llit. The dilfcl'cnecs , hcy show, howcl'er, are not greatcr but, on thc contrary, cvcn
less than Ihose found " mo, ig- si, tcr hyb,' ids.
Fig". 7. - Cill'allge, :';, 4305a ( Cill" lls
Il'i!" ol iala cL'os:; ed lI" i ill L1Le Thompson
~ aYcl orange); sholl" s many 5- folialc
Icaycs. Aboul 1/ 5 lIallwal sizc.
W. [ C. SWE'iGLE. - VARLlTIOJ'i IN FIRST GENI~ HA' 1' JOX HYBRIDS. 381
Enough has been sa id to sho\\' tba t Lhere' are striking variations among
first- generation hybrids 01" citrous fruils.
Messrs. Collins and Kempton', in an nrticle presented to this congress,
havG given conclusive evidence of the varying hereditary nature of dill'erent
individual plants of first generation plants of maize, alLhough tbese difTerences
\ yere indicated by the nature of the endosperm iLself representing F2 , or the
pcrjugategeneration. A Chinese maizewith
a waxy endosperm was cro~ sed \\' jth Mexican
maize with a horny endosperm of the usual
type. The homy endosperm was dominanL
so all the kernels on the crossed cars sho\\' ed
the horny character. These kernels sowed
separately gave ri,; e to plants of the first
generation ( F I ) which \\' ere self- pollinated.
The resulLing cars shO\ yed varying percenta­ges
of kernels with a waxy endOSpel'l11 from
15.7 to 53.5, approaching nearly but not quite
to the expecLed Mendelian ratio 1- 5 ( being 25.' 1
instead of 25 per 100).
In as much as the ears contained a large
number of kernels there can be no doubt but
that their varying percentages represented
rcal difTerences in the hereditary composition
of the first generation plants. It would be
hard to find a more conclusive case since
there could be no doubt as to the purity of
the parents and whatis more rare no possible
doubt as to whether a given kernel had a
waxy or a horny endosperm.
Such variations among individuals of
first generation hybrids are not unknown to
students of genetics amI these phenomena
have been referred to variable dominance by
those who use the ~ Iendelian terminology.
It has not, however, been realizrd how fatal
this phenomenon is to some of Lhe chief te­neLs
ofmoa. ern theories of hered i Ly. Already
in J897 at the Ithaca meeting of the American
Association for the Advancement of Sciencp,
I called attention to this phcnoinenon and, to
account for it, proposed a provisional hypo-thesis
of \\" hich a summary was given in the following words:
" IL \\' as pointed out that \ Veismann's theory of reduction of chromosomes,
though giving a plausible explanaLion of the dj( Terences observed between the
first ( unil" orm) and second ( polymorphic) generations of most hybrids, is not
only in disaccord with the observed phenomena of spore and pollen formation
in higher plants. but fails to account 1" 01' the extreme polymoiphism often
' 1. CollillS, G. N.. and J( cLllplon, .1. 11., oJ\ J12, InhcL'iiancc of \\ axy Endosperm ill lIybrids of ChiLlcsc
COl'll, in thesc l'I'Oi'el'dillgs, p. 547.
588 IV · CO~ FERE;,\ CE l.\' TER" ATlO.\' ALE OE GlbETiQCE.
Fig. 8. - Cilremoll. ;\ 0 4( jOiO ( Li5bon lelllon crossed wilh Ull'lIs ll'i( oliala).
5howing leaves wilh very small side leallels; an unusual lype, seldom
found in cilranges. J/? nalUral size.
observed in the first generation of hybrids LeL\ yeen races of cuILivated plants,
or bet,, · een closely
reI a led species, as
for example some
racial hybrids of
maize and some
speci fie hybrids
of Lychnisand Di­gitalis.
i\ Ir. S\ yin­gle
considered it
necessary lo as­sume
in some
su ch cases, at
least, a predeter­mination
of the
characters of the
hybrid al the Lime
of fusion of the
male and female
nuclei. The male
and female chro
mosomcs proba­bly
persist side by
side unchanged
in number. and
possibly unchan­ged
in quality du­ring
Lhe whole of
the onLogeny of
the hybrid, reduc­tion
noLoccmring
until the close or
Lhe first genera­tion.
It is the­refore
necessary
Lo assume, in or­der
lo explain Lhe
observed fact of
di vergence ofcha­racter
in Lhe first
genera tion of so­me
hybrids, thaL
theinfluenceexer­Led
during onto­geny
of the hy­brid
by the mate­rial
bearers or
heredity IS, aL
least in some ca-
\\'. S. SWl~ GLE. - VAnL\ TLO~ I:\' FIRST GENERATION HYBRIDS. 58!)
r
· 1
ses, a function of their relative positions; and flll'ther, that in most cases the
Fig. H. - Citl'cJnon, j\, 46351 ( Lisbon lemon Cl'Ossccl willt Git1' lts tn/ ahata), showing tl'ifolialc lcarcs
"' ith vcry large lateral leaflets as in citrangcs. -'/ 2 natural size.
•
I'elalive positions of these bearers of hCl'edity, as determined at lhe moment' of
fusion of the male and female nuclei, persist unchang'ed throughout ontogeny
of the offspring. Some phenomena, such as reversions to the one 01' the other
parent form by a larger or smaller portion of the hybrid, \," ould be explained by
3\ 10 1\' 0 CO.'\ FI~ nE~ CI\ I.'\ TgR;'\.\ TIO.' i\ L1~ OE G~.' iI;: TIQlm.
assuming some change in llw di" po,; ilion of llw hrrrdilary substance, whereby
!'" ig;. Ill. - Lisbon LClIlon. ~" " li" i21. Il, c ,-; I,' iCll' lIsed in Ill: ll; illg; Ihe hybrids CilIY'lllons) illusll'; lled
in I · \; s. 5.8 and 0. ~ h() ws roh: lg · c. 1/ 2 n31ul'al size.
they assumed a ne\\" position 01' parlial 01' comp! rte stability. The suggestion
was made that possibly the difl'erence between uniform and pol~" morphic hy-
• I
w. S. WINl; L1~. - \' AHIATIO~ I~ FIRST GEXEnATJO~ UYBRIDS. 391
J
brids of the fhst generation is due to a more complete intermingling of the
heredi tary particles in case of polymOl'phic hybrids ( oflspring of closely related
organisms), "' hereby many difTering combinations would be possible, and in
case of uniform hybrids ( mosLly offspring of distinct species or very difTerent
races of the same species), to greater or less aversion to commingling between
the " YO more diverse sorts of particles, "' hereby lhey would remain in two
separate groups and afl'ect ontogeny unifOl'mly and eqnally',
Since this announcement "' as made some e" idence has been published' to
show not only that ind ividual chromosomes persist through the so- called resting
stages of the nucleus but also tha t the particular con figuration is preseryed
from. one cell generation to another wi th some accu racy.
JL seems nol. improbable that this theory of a positional 01' vectorial
inl1ul'llce of chromosomes may be fUI, ther amended hy assuming that the
chl'Omosomes which happen to lie nearest the nuclear wall and, ex hypothesis,
thereby exert [ he grealpst influence on lhe charactrr of the cells containing
them, are likeWIse beller nourished than the chromosomes located neal' the
center of the nucleus which al'e thereby prevented both from exerting their
full measme of influence on the developing cell and from receiving the most
abundant nomish menL. Il migh t be su pposed that the \ yell nou rished chromo­somes
\ yhich from their superficial position onthe nucleus had been preponde­rant
in determining the charncter of the individual , yould also dominate during
synapsis and lend to give as a resulL gametes similar in their hereditary
c ha racter to the first general ion hybrids whose character \ yas determined by
the configuration 0[' the chromosomes \ yhieh accidentally came about at the
moment of fusion between the nuclei of the syngamete,
. Thme types of nuckar configmaLion mighL be assnmed to oc'cur in higher
organisms. If a violent cross be made, very unlike species being hybridized,
the paternal and maternal chromosomes might repel each other and occupy
opposite sides of the syngamete nucleus, exerting equal influence upon [ he
developing organism and as a consequence the first generation h)' brids be" yeen
widely distinct species are often but very slighLly \' llriable and are almost
stricLly intermediale between the two parents, Such hybl'ids are uS1Jally sterile,
that is to say, the chromosomes are so unlike that synapsis ( mitapsis) cannot
occur The mule is a striking example of this class of hybrids, which are also
common among plants,
The other extreme is seen when abnOl'mally inbred races of domesticated
animals and plants arc crossed, Such forms are likewise usually intermediate
in the first generation and are fertile, the nuclei being able to pass lhrough
synapsis and form fertile gametes. The synapsis, however, is more or less
inperfect and the patemal and maternal idioplasmic particles may not, propel'ly
speaking, fuse or blend at all but go over more or less unmixed to the resuILing
gametes. Possibly because of long continued narro,," breeding and resulting
great amnily for a non · related mate, the pail'S of chl'Omosomes might remain
juxtaposed in Ihe first generation, preyenting much vectorial influence during
the first generation. The dominance of cel'lain charadel's which is regularly •
1. Swinglc, \ Y. T, IS09. Somc thcorics of hcrcdity < Ind of thc oril-( in of ~ pccies con- idcrcd ill I'ela­tion
to thc phcnomcna of hl'b,' idization [ AbslI.. , ctJ, in Ihtallical C( l:;, ettC. 2: i: 111- 113 ( n. 2. Feb.).
2. 1' 01' cxamplc Iloycri, Th, odo,', H/ Ol. J: l'gcbllissc iibel' die liolls/ it', lio/ l del' c/,,' oli/ alisc! tI' 1I SlIb­,[
al/:;' des Zelll, em" pp. o- ll. fig. Li-! J. Jcna . Fischcr).
592 lye CONFERENCE INTERNATlONALE DE GENETlQUE.
observed in Mendelian hybrids is due rathcr to the natme of the chromosomes
from thc diflerent parcnts than to their relative position in thc syngamic nuclci.
The third condition, probably normal in wild spceics, is that in which the
chromosomes freely interminglc at the moment offccundation, the configmation
they then takc on accidentally being pcrpetuated throughout the life of the
individual. Thc difl'crent chromosomcs exert varying degrees of influence on
the cell, depending on whether thcy arc lucated near the periphcry or near the
center of the nucleus' .
The following synopsis exhibits the three types of fccundation.
I. INTU1SPECIFIC IIYBI1IDS : Usually stCl'ile and intcrmediate.
Violent crossbrecding. Chromatic material tends to remain separate;
synapsis often impossible, resulLing in sterility.
II. MENDELIAN CROSSES: Usually intermcdiate, dialytic at synapsis.
Mendelism, crossing of inbred hybrids with imped'ect or false synapsis.
III. l\ ORMAL CROSS- Bl1ED sPEciES: Usually vigorous, fertile and variable.
Normal crossbreeding. Chromosomes frcely' intermingle at moment ot
fecundation and the configuration there taken persists through the ontogeny
of thc individual, giving great variety of forms. Synapsis normal.
As 10 the fact of considerable variations in first gencration hybrids of rela­Lively
stable species or races there can bc no doubt.
In the light of what has becn said above, it cannot be doubted that we have
in such variations a vital and important principle of heredity and one which
goes far to ~ xplain many obscure phenomena of the everyday cxpericnces of
animal and plant breeders.
The suggcstion, first made in ' 1897 and here rciterated, Lhat such variation
may, in part at least, be due to positional dif1' erences in the material bearers of
herediLary characters is of coursc merely a provisional hypoLhesis As such it
may prove hclpful in gelling a clear idca of the phenomena to be explained
even if it does not aid us materially in such an explanation.
Cook · has pointed out that this theory, of positional relations of the idio­plasmic
materials of the cell, puts a ncw rcsource in the hands of the evolu­tionist
in the study of the bafflingly complcx expression rclations of hereditary
chal'acters.
It is easy Lo see that if' ,,- e accept the hypothesis of thc positional relation
of chromosomes, a very large number of different forms could be expected to
occur among hybrids ' of Ihe same relatively pme- bred parents, In this \ Yay Lhe
practice of almost all successful plant breeders in selecting the best individuals
f'rom among a large numbcr of hybrids receives a new justification,
] n order to have a name for this supposed positional influence of the
parcntal idioplasm in the cells of cl'Oss- bred individuals, I propose the term
;:, ygotaxis.
By zygotaxis:; \ Vc mean the arrangement in- thc ;.: yngamete ( zygote) of the
I. The law of liIial I'cgrcssioll discol'cl'cd by Frallcis GaitI'll ( Satural lllhcritaucc, Macmillan Co"
ISS!). PI" !) 5, ct scq.) may in part be cxplaillablc as I'cprcsclltillg tl, C mC; JSll1' C of t. he Cxlpl1l to ", Jlicl'
th'esc accidental conllguraliol1s are ", iped alit dUl'ing' tllC succccding syllapsi,. If such cOllfig'ul'alions
,,- el'C completcly wiped oul, Jilial rcgTession would bc normally 100 % instead of somclhing likc
;; 5 % as found by Galton.
~, Cook, O. F., ' 1\ HO. ~ lulaliYe revel'sions in colton, Circnlar II 53, 1I. 1'. I., U. 5, Dcpt. AgTic., p. ' I;;
~ Iou · . 21).
j. From ~' Jyi,' 1. yokc ( for zygote), and ~ c1,~,;, alT, Jllg'emcnl, with referencc 10 thc faci 111nt such all
_~_~_~_~__ ~.- o,,- _
\ r. S.\ H\" GLE. - YARIATlO:\: IN FIBST GE.' EBATlO:\: UlBRIDS.
chromatin and other hereditary substances derived fl'Om the parental gametes
and the persistence of this arrangement in the cells produced by Lhe subdivi­sion
of the syngamete. It is assumeu lhat the particular zygotactic anange­ment
taken up by the chromosomes of the parental gametes usually persists
"' ith lillie or no change thmughout Lhe lifr of tbe organism. It is easy to sec,
however, that many bud Yariations, sports, and other mutations could be
explained by zygoLactic changes occurring during ontogeny.
It must be remembered that the effects pJ'Oduced by zygotaxis are usually
obscured by variaLions due Lo Lhe v31' ying hereditary composition of the gametes
and only rarely when very constant races or "' idely different species arc crossed
can we see variations due Lo zygotaxis alone.
This does not mean that zygotaclic effecLs arc rare, on the conLral'y they
probably occur as conLributary causes of varia Lion in much if not all of the
maLerial now being investigaLed by students of heredity.
VARIATION EN I'" GENERATION CHEZ LES HYBRIDES ( DOMINANCE IMPARFAITEj:
SON EXPLICATION POSSIBLE PAR MOYEN DE ZYGOTAXIE
REsmIE
I. Les hybrides enLl'e Ie Cil,' us ll'i( ulirlla et ks autl'es especes unifoliolces
de CiU'us comme I'Ot'anger, Ie limonier, Ie « gl'apefl'l1it » , etc., montrent une
grande etendue dans les variations a la premiere generation. Ceci devienL
evident quand on compare les hybrides issus des graines d'un · meme cl'Oise­menl.
Dans quelques cas, les especes parentes bien connues pour ne pl'Oduire
que de legeres variations par Ie semis, donnent, dans les croisemenLs, des F j
aYe~ variations bien plus eLendues.
2. Ces variations sont tres apparenLes chez les fruits qui different beau­coup
les uns des autres pal' la taille, la couleur, la Lexlure, Ie gOiH, la pubes­cence
cLmf\ me par leur constitution, QueIqurs hybrides dans Ie meme semis
presentent des fruiLs reguliers tandis que d'auLres ont des carpelles indepen­dants
( carpolysis). Ou encore une superf61aLion proJuisant un fruit a deux
Mages, Ie second fruit (' Iant parfois saillant et meme separe par un axe coml.
Dans Ie cas d'hybrides (' ntre Ie citronnier et Cilnls ll'i( o{ iala, on obsen'e encore
une grande variation dans Ie feudJage qui, dans les sujets issus en premicre
generation des meme~ parents, cst constiLue chez les uns pal' des feuilles uni­foliolces,
chez les auLres pal' des feuilles Irifoliolees ou encore, chez cerlains,
par des feuilles bl'act6ifonnes ( hypophylles).
J. Puisque les chromosomes deriycs des gamCles parents persisLent cote a
cote, sans se fusionner d urantla Lotali Le de la general ion conj uguee 1" 1' de telles
variaLions obsen'ees dans les - hybrides ne peuyent pas eire dues a des difle­rences
quanLiLatives ou meme qualiLatives dans les faclems de I'hcredite pos­"
edes par les hybl'ides freres issus des graines d'un seu] fruit.
arrang- cmrnl of lhc <: l1l'olll< ltinIl'OlIld bc mosl likcly 10 OCCIII' al Ihc Illomcill uf fll, ion ~ lhe IWl'elllal
g" amCleS 10 form lhr zygOlC. The fundamcntal idra undcl'lying thc tcrm z~" p; otaxis is thal lhe architcc­IlIrc
of Ihc zygotc , rilh I'cfercncc to ils idioplasmic pm �� ticlrs, as well as ils meclwni;; ms fOI' II'allsmit­tmg
hercdital'y tcndcncics inlo expl'c-;; ion, is dctcrmincd to SOIllC exlcnl al thp mOll1cnl or fusion of
Ihc two pal'cnlal gametcs all I thal Ihis pal'liculal' al'l'angcmcnlof pal'l,: is tram, millcd to thc cclls of
thc ol'ganism to "' hich the zygotc gircs risco
5! l4 1\' · CO.\' FEHE:\: CE !~ TER.\' ATIO~.\ LE DE GE~ ETIQUE.
1-. En 1897, I'aulcur a propose I'hypoLhese sui,- anlc : de Lelles varia Lions
dans les hybridcs freres ( F l ) scraicnl dues it dcs difrerences dans les positions
relalives dcs chromosomes cl d'auLres facleUl's de l'heredile derives des gameLes
parenLs cl alors l'arrangemcnl parliculier pris d'une maniere plus ou moins
accidcnlcIle, au momcnt de la fusion des noyaux, pcrsisLe pendant Loule
I'cxislcncc dc la prcmiere generalion.
J. II peut Nrc pris en consideraLion quc les chromosomcs qui arrivent a
eIre places pres de la peripherie du noyau onL cxcrce une plus grande influence
sur Ie developpement dc l'hybridc que ccux qui occupenL une posilion pIns
ccnLralc.
II peul encore eire admis que les chromosomcs peripiJeriljUeS sonL micux
noun'is par Ie lrophopJasme qui les enyironne el tcndenl cn consequence a
domincr les chromosomes ccnLraux, plus faibles, pendanlla fusion mitapliquc
( synapsis) et imprimenl unc derivalion.
Ccllc inl'gaJile dans la vigucur des chromosomes, due a l'influence de Icul'
posilion cl a ]' inegaliLe dc la nourrilUl'c qu'ils re~ oivcnt peut elre admisc
comme ayanl unc influencc sur la seconde generalion cl les sui\ anLes cl
expliquc peuL- elre quelques devinlions des regJes de :\ lenJcl.
6. Les rclaLions de posiLion dcs chromosomes prises au momenl de la
fusion des gamelcs cl qui pcrsisLrnl pendanl Loule la durec de la generaLion
conjuguee ( Fll esl appclee ::; ygolaxie. CcLle zygolaxie pourra produire les varia­lions
observees chez les hybrides de la premiere generaLion ( Fll donL les parenLs
donnenl chacun, rcgulieremenL, Jes gameLes uniformes.
7. La zygoLaxie esl d'unc grande imporlance dans l'obLention des varieles
nou " dles en assu ran I des com binaisons uLiles des carad(\ res deri ves dcs dcux
parenls. Ellc poul Nrc la basc scienlifiquc de la produclion en grand nombrc
d'hybridcs conjugucs F l , pralique suivie par beaucoup d'hy! JrideUl's.
8. Le resullal dcs varia lions zygolaxiques oldenues en croisanL l'orangcr
avcc Ie Citrw; lJ" ijoliala esl un nombrc considerable d'hybrides appeles
ciI1' an[ jl's, nouveau lype de fruiL. Ccs cilranges monLrenl un lres grand nombre
de varialions, el quclques uns d'enlre eux, nolammcnl le lolman, Jlorlon eL
Busk sonL lres jUlcux cL fincmenl parfumes. Ces fruiLs, Lres acidcs, peuvenl
rempJaccr les citrons, et etrc culLi\"(~ s dans des I't'gions assez froides aLI aucune
aulrc cspecc de Cilru8 comeslible nc peul d'sisler.