Savidan, Yves

Horizon / Pleins textes

Theor. Appl. Genet. 57,153-156 (1980) 
@ by Springer-Verlag 1980 
~ 
Chromosomal and Embryological Analyses in Sexual x Apomictic 
Hybrids of Panicum maximum Jacq. 
Y. Savidan 
ORSTOM Research Center, Abidjan (Ivory Coast) 
Summary. Cytological analyses in series of crosses be- 
tween 7 sexual pistillate and 8 apomictic staminate par- 
ents of species Panicum nzaximunz (Gramineae) are report- 
ed. Although these 15 progenitors were tetraploid (2n = 
32), 2 dihaploids (2n = 16), 45 hexaploids (2n = 48) and 5 
octoploids (2n = 64) were observed among 333 progeny 
plants. The role of unreduced gametes as the originators 
of polyploidy is discussed in relation to the so-called 'ele- 
ments of apomixis'. The 2 dihaploids appeared to be sexu- 
al while the hexaploids and octoploids were all apomictic. 
At the tetraploid level sexual and apomictic hybrids segre- 
gated in a, ratio close to 1: 1. These results were then com- 
pared to those already obtained from studies on other 
tropical grasses and indicate a simple genetic determinism 
for gametophytic apomixis. 
Key words: Panicum maximum - Embryology - Apo- 
mixis - Sexuality - Polyploidization 
Introduction 
The tropical forage crop Panicum maximum Jacq. (Grami- 
neae) forms an agamic complex of great economic value. 
It is mostly represented in nature by tetraploid biotypes 
with 2n = 32 chromosomes that reproduce by facultative 
apomixis. Obligate sexuality was first discovered in small 
natural diploid populations of East Africa by Combes and 
Pernes (1970) and subsequently at the tetraploid level, in 
progenies of natural apomicts from South Africa, by 
Smith (1972) and Hanna et al. (1973). 
Crossing sexual tetraploids, obtained from colchicine 
treatment of the natural diploids, with apomicts has been 
attempted since 1971 in the Ivory Coast. Preliminary re- 
sults were discussed in previous reports (Pernes et al. 
1975a; Savidan 1975, 1978). The major objectives of this 
paper were to study the chromosome numbers and fhe 
inheritance of modes of reproduction, apomixis and sexu- 
ality, in progenies from series of crosses between seven 
sexual and eight apomictic tetraploids. 7 
Materials and Methods 
More than 500 accessions of Panicum maximum, mostly from 
Kenya and Tanzania, are kept in a field collection in our Research 
Center (Abidjan, Ivory Coast). Only 22 of them are diploid and 
entirely sexual. The origins of the different genitors used in our 7 
X 8 series of crosses are shown in Table 1. 
Cytological methods were preferred to morphological tests for 
the study of modes of reproduction as they were found to be 
more convenient and more rapid. The technique using phase con- 
trast microscopy has been described and analysed in previous 
papers (Herr 1971; Savidan 1975). However, field tests were peri- 
odically conducted as controls. Sexual autotetraploids are mostly 
self-sterile and exceptional self-fertilized products are morphologi- 
cally well-differentiated from hybrids. As a result we used the 
following hybridization technique: two inflorescences - one from 
each parent - were put together in a paper bag and seeds were 
collected on the female panicles. A minimum of 10 bags were set 
for each cross combination. Chromosome numbers were observed 
after fixation in Carnoy (6:3:1) and hematoxyline coloration. The 
sexual or apomictic behaviour was studied by observing the struc- 
ture of twenty ovules from each hybrid. 
Results 
Ten crosses didn't produce any hybrids and conspicuous 
differences were observed involving both success of hy- 
bridization and ploidy levels (Table 2). 
1) Success of hybridization: under our experimental 
conditions, IS4 was the most productive sexual genitor 
ploid T34.T. This, along with the 10 crosses that didn't 
produce any hybrids could have resulted from differences 
and yielded ten times more hybrids than the autotetra- 
154 
originated, was known to yield higher amounts of seeds 
than any other diploid of our collection. The numbers of 
hybrids obtained from each apomictic tetraploid was not 
reported here since no apparent differences were shown. 
2) Ploidy levels: fifty-two of the 333 plants produced 
had a ploidy level different from that of the parents, i.e. 
2n = 4x = 32. 
Two dihaploids (2n = 16) were vigourous and fully 
fertile. They looked morphologically very similar to our 
natural diploids. As no characteristic from the staminate 
parent could be detected, one can assume they arose from 
parthenogenesis. It must be noted that 60 dihaploids were 
previously observed in Panicum maximum (Pernes et al. 
1975b). They arose from a presumed natural interspecific 
hybrid between P. maximum and P. infestum. In spite of a 
regular meiosis, with eight bivalents, all of them were fully 
sterile. Further embryo sac analyses showed they were 
potentially apomictic. 
Forty-five hexaploid hybrids (2n = 48) were observed. 
They probably resulted from fertilization of unreduced 
' 
Table 1. Origins of the fifteen genitors. Accessions preceded by 
letters K and T were brought back from two trips to Kenya and 
Tanzania in 1967 and 1969. The apomictic group used as staminate 
parents is highly polymorphic 
Sexual genitors 
T34.T 
T44.T 
K189.T same origin 
S2.T 
Vupiri, Tanzania, diploid treated with colchicine 
Korogwe, Tanzania, diploid treated with colchicine 
seed from self fertilization of diploid K189, treated with 
colchicine 
sexual hybrid from a K189.T X G23 cross, G23 being an 
. introduction from Kenya 
sexual hybrid from a K189.T X G3 cross, G3 being an 
introduction from Nigeria 
P2 
IS3 
IS4 Full sib of IS3 
Apomictic genitors 
K211 
G17 
K4 
c1 
280 
T25 
K26 
K20 
south of Mombasa, Kenya 
introduction from Kenya 
Nairobi, Kenya, a presumed natural interspecific hybrid 
with species Panicum trichocladum 
spontaneous tetraploid appearedin a collection of sterile 
dihaploids from presumed natural interspecific hybrids 
with species Panicum infestum 
introduction from Kenya 
Bagamoyo, Tanzania 
Between Rumuruti and Maralal, Kenya 
Between Rumuruti and Thomson's falls, Kenya 
Theor. Appl. Genet. 57 (1980) 
egg cells. They grew vigorously and some of them were 
selected for agronomic interest. The 5 octoploids (2n = 
64) were obtained from the same staminate parent, i.e. 
C1 , which is presumed to be a natural interspecific hybrid. 
Embryological studies indicated that dihaploids were 
fully sexual while hexaploids and octoploids were all 
apomictic. In tetraploid hybrids, including aneuploids 
with 30, 31 or 33 chromosomes, reproduction was either 
sexual or apomictic. Table 3 shows the segregations ob- 
served in the tetraploid progenies. Isogenic artificial sexual 
tetraploids are grouped on one side and the three sexual 
hybrids on the other. A segregation close to 1:l  is ob- 
served in the F1 hybrids as well as in the three-way hy- 
brids. It should be pointed out that among the 281 tetra- 
ploid hybrids analysed, 141 were sexual and 140 apomic- 
tic. 
Discussion 
Our experiments produced new information at three lev- 
els: (1) the level of unreduction in sexual plants, as related 
to polyploidization, (2) the relation between unreduction 
and embryo sac structure, and (3) the inheritance of 
apomixis, in Panicum maximum. 
Unreduction and Polyploidization 
In a review on the origin of polyploidy, Harlan and De 
Wet (1975) mentioned 68 genus in which hybrids obtain- 
Table 2. Numbers of hybrids from each sexual genitor and their 
ploidy level. The difference between rates of chromosomal abnor- 
malities in progenies from (1) isogenic sexual tetraploids, and (2) 
sexual hybrids, is highly significant (x' = 38.95) 
2n = 16 32 Aneu 48 64 Sum 
~~~ ~ ~ ~ 
T34 .T O 5 1 3 O 9 
T44.T O 2 1 13 1 17 
K189.T O 25 6 2 2 35 
S2.T 1 32 4 14 O 51 I 
P2 O 35 3 4 2 44 
IS3 1 70 4 8 , O  83 
IS4 O 86 7 1 O 94 
SUm 2 255 26 45 5 333 
32 + Sum 
(1) 64 48  112 
(2) 191 30 22 1 
Sum 255 78 333 
Y. Savidan: Chromosomal and Embryological Analyses in Hybrids of Panicum iizaximuin Jacq. 155 
Table 3. Numbers of tetraploid hybrids (including aneuploids) 
for each class of sexual genitors, with their apomicticJsexual ratio, 
tested for a 1 to 1 segregation 
~~~~ 
Female parents n APO SEX x2(1:1) 
Isogenic sexual 
tetraploids 16 45 31 2.58NS 
Sexual hybrids 205 95 110 1.10NS 
ed from the fertilization of unreduced egg cells were ob- 
tained. These authors thus claimed that polyploidy did 
not especially result from chromosome doubling after 
hybridization, as stated by Stebbins (1950), but from fer- 
tilization of unreduced gametes. The production of such 
gametes were seemingly strongly correlated with remote 
hybridizåtion. Any sexual plant could produce abnormal 
gametes since meiosis is never hundred per cent successful. 
In our preliminary experiments (Pernes et al. 1975a; 
Savidan 1975), we did not see such chromosomal abnor- 
malities, perhaps because the parents of the crosses were 
not as distantly related as those used in the new series of 
crosses. The choice of the eight apomictic staminate par- 
ents in these crosses can itself explain the occurrence of 
50 hybrids with a 2n + n or 2n + 2n structure. The rate of 
unreduction (15.0 per cent) is quite similar to that ob- 
served in other species of the Bothriochloa-Dichanthium 
complex (Harlan and De Wet 1975). 
Unreduction and Embryo sac Structure 
As unreduced egg cells can result from apospory or diplo- 
spory, hybrids coming from fertilization of unreduced 
gametes are often looked upon as a sign of a ‘tendency 
towards apomixis’; the unreduction process being regard- 
ed as a socalled ‘element of apomixis’. (Grober et al. 
1974; Petrov et al. 1978). Thus it does not always seem 
easy to separate apomixis from a ‘sexual polyploidization’ 
process, as defined by Mendiburu and Peloquin (1976). 
Our experiment perhaps brings a new element to the dis- 
cussion. 
Panicum maximum, like other facultative aposporous 
Panicoideae, is characterized by a gametophytic dimor- 
phism. Embryo-sacs of aposporic origin, which are the 
more frequent, are 4-nucleate: one egg, 2 synergids and a 
unique polar nucleus. Embryo-sacs of sexual origin are 
8-nucleate. They have antipodal cells and 2 polar nuclei 
(Warmke 1954). Numerops cytological observations were 
made on the T44.T genitor, as it had a high rate of poly- 
ploid hybrids production (14 among 17 plants, see Ta- 
ble 2). Several hundred ovules showing only 8-nucleate 
embryo-sacs were observed. Our suggestion is that unre- 
duction in the sexual polyploidization process and un- 
reduction in the apomictic process could be differentiated 
as they are related to two morphologically different,types 
of embryo-sacs in such material. In the former case, fol- 
lowing meiosis, unreduced cells are produced that develop 
into a 8-nucleate structure; in the latter case, all mega- 
spores abort and are replaced by a nucellar cell that de- 
velops into the typical 4-nucleate structure. 
We are now convinced that two different types of un- 
reduction exists in plants, one of which is purely sexual. 
That is why we assert that looking for apomixis in hybrid 
progenies that are partly composed of 2n t n products is 
absolutely illusive. Apomixis, as defined by Gustafsson 
(1946) and accepted by almost all specialists, is the result 
of both unreduction and unfertilization. Therefore one 
has to be careful in using the term ‘element of apomixis’. 
This term should be abandoned. 
Inheritaitce of Apomixis 
Studies conducted since the 50’s on such Gramineae as 
Paspalum iiotatum (Burton and Forbes 1960), the Both- 
riochloa-Dichanthium complex (Harlan et al. 1964), 
Cenchrus (= Pennisetum) ciliaris (Taliaferro and Bashaw 
1966; Read and Bashaw 1969) and Eragrostis cumula 
(Voigt and Bashaw 1972) suggest that one should recon- 
sider the agronomic interest of gametophytic apomixis, at 
least for aposporous Panicoideae (Savidan 1978). Almost 
all forage and grain crops of intertropical areas belong to 
this subfamily. Two improvement programmes have al- 
ready been successful in these plants, one in the genus 
Cenchrus (Bashaw 1975); the other in Panicum maximum 
(Pernes et al. 1975~;  Savidan 1978). Two other examples 
of a simple genetic determinism were recently given, one 
in the diplosporic Tripsacum dactyloides (Gramineae) by 
Petrov et al. (1978), the other by Nogler (1978) in a 
dicotyledon: Ranunculus auricoinus. 
The determinism of apospory in Panicum maximum 
appeared to be simple, although not yet completely ana- 
lyzed. Anyhow, there could be more than a simple allelic 
dosage for the expression of apomixis. Since no meiotic 
abnormalities have been shown in the staminate parents, 
the hexaploids certainly received twice as much informa- 
tion from the sexual pistillate parent than from the apo- 
mictic staminate parent. In spite of such a convenient 
allelic dosage, sexuality can not be expressed in these 
hexaploids. 
Our crosses especially demonstrated that apomixis and 
sexuality could be easily managed in an improvement pro- 
gramme, as two and three-way hybrids of Panicum maxi- 
mum were produced. These hybrids had a low rate of 
sexuality when apomictic and good grass productivity 
(Savidan 1980), and therefore could be of special com- 
mercial interest. 
156 
Acknowledgement 
I thank MI. E. Anon Amon for his technical assistance, Prof. 
J. Pernès and Dr. A. Charrier for their helpful comments and dis- 
cussions. 
Literature 
Bashaw, E.C. (1975): Problems and possibilities of apomixis in the 
improvement of tropical forage grasses. In: Tropical forage in 
livestock production systems. ASA Special Publ. 24,23-30 
Burton, G.W.; Forbes, I. (1960): The genetics and manipulation of 
obligate apomixis in common Bahiagrass (Paspalum notatum 
Fliigge). Proc. VIIIth Intern. Grassl. Congr. pp. 66-71 
Combes, D.; Pernes, J. (1970): Variations dans les nombres chro- 
mosomiques du Panicum maximum Jacq. en relation avec le 
mode de reproduction. C.R. Acad. Sci. (Paris) sér. D 270, 
Gröber, K.; Matzk, F.; Zacharias, M. (1974): Untersuchungen zur 
Entwicklung der apomiktischen Fortpflanzenzungsweise bei 
Futtergräsern. I. Art and Gattungskreuzungen. Kulturpflanze 
Gustafsson, A. (1946): Apomixis in higher plants. 1: the mecha- 
nism of apomixis. Lunds Univ. Arsskr., 42,l-68 
Hanna, W.W.; Powell, J.B.; Millot, J.C.; Burton, G.W. (1973): 
Cytology of obligate sexual plants in Panicum maximum Jacq. 
and their use in controlled hybrids. Crop Sci. 13,695-697 
Harlan, J.R.; Brooks, M.H.; Borgaonkar, D.S.; De Wet, J.M.J. 
(1964): Nature and inheritance of apomixis in Bothriochloa 
and Dichanthium. Bot. Gaz. 125,4146 
Harlan, J.R.; De Wet, J.M.J. (1975): On O. Winge and a prayer: 
the origins of polyploidy. Bot. Rev. 41, 361-390 
Herr, J.M. (1971): A new clearing squash technique for the study 
of ovule development in angiosperms. Amer. J. Both. 58, 
Mendiburu, A.O.; Peloquin, S.J. (1976): Sexual polyploidization 
and depolyploidization: some terminology and definitions. 
Theor. Appl. Genet. 48, 137-143 
Nogler, G.A. (1978, in press): Zur Zytogenetik des Apomixis bei 
Ranunculus auricomus. 
Pernes, J.; Combes, D.; ReneChaume, R.; Savidan, Y. (1975a): 
Biologie des populations naturelles du Panicum maximum 
Jacq. Cah. ORSTOM, s6r. Biol, X (2), 77-89 
78 2-785 
XXII, 159-180 
785-790 
Theor. Appl. Genet. 57 (1980) 
Pernes, J.; Savidan, Y.; Rene-Chaume, R. (1975b): Panicum: struc- 
tures génétiques du complexe des ‘Maximae’ et organisation de 
ses populations naturelles en relation avec la spéciation. Bois- 
siera (Genkve) 24,383402 
Pernes, J.; ReneChaume, R.; Rene, J.; Savidan, Y. (1975~): 
Schéma d’amélioration génétique des complexes agamiques du 
type Panicum. Cah. ORSTOM, sir. Biol., X (2), 67-75 
Petrov, D.F.; Belousova, N.I.; Fokina, E.S. (1978): The inheri- 
tance pattern of apomictic elements in interspecific hybrids 
between maize and Tripsacum. Proc. XIVth Intern. Congr. 
Genetics II, 184. (Abstr.) 
Read, J.C.; Bashaw, E.C. (1969): Cytotaxonomic relationship and 
the role of apomixis in speciation in Buffelgrass and Birdwood- 
grass. Crop. Sci. 9,805-806 
Savidan, Y. (1975): Hérédité de I’apomixie. Contribution B 1’6tude 
de l’hérédité de I’apomixie sur Panicum maximum Jacq. (ana- 
lyse des sacs embryonnaires). Cah. ORSTOM, sir. Biol. X (2),  
Savidan, Y. (1978): L’apomixie gamétophytique chez les Grami- 
nées et son utilisation en Amélioration des Plantes. Ann. Amé- 
lior. Plantes 28 ( l ) ,  1-9 
Savidan, Y. (1980, in press): Facultative apomixis as a tool for the 
improvement of Guinea grass (Panicum maximum Jacq.) 
Smith, R.L. (1972): Sexual reproduction in Panicum maximum. 
Crop Sci. 12,624621 
Stebbins, G.L. (1950): Variation and Evolution in Plants. New 
York: Columbia Univ. Press 
Taliaferro, C.M.; Bashaw, E.C. (1966): Inheritance and control of 
obligate apomixis in breeding Buffelgrass, Pennisefum ciliare. 
Crop. Sci. 6,473476 
Voigt, P.W.; Bashaw, E.C. (1972): Apomixis and sexuality in Era- 
grostis curvula. Crop. Sci. 12,843-847 
Warmke, HE.  (1954): Apomixis in Panicum maximum. Amer. J. 
91-95 
BOt.41,5-11 
Received October 20,1979 
Accepted January 28,1980 
Communicated by W. Gustafsson 
Dr. Y. Savidan 
ORSTOM Research Center, 
B.P. V51 
Abidjan (Ivory Coast) 


Chromosomal and Embryological Analysis in Sexual Apomictic Hybrids of Panicum maximum Jacq.

Abstract

Similar works

Full text

Available Versions

Horizon / Pleins textes