Euphytica 101: 79–82, 1998. 79c 1998Kluwer Academic Publishers. Printed in the Netherlands.

Segregation of bunch orientation in plantain and banana hybrids

Rodomiro Ortiz1 & Dirk VuylstekePlantain and Banana Improvement Program, International Institute of Tropical Agriculture, High RainfallStation, Onne, PMB 008, Nchia Eleme, Rivers State, Nigeria;1 present address: The Royal Veterinary andAgricultural University, 40 Thorvaldsensvej, DK 1871, Frederiksberg C, Copenhagen, Denmark

Received 7 April 1997; accepted 22 September 1997

Key words:auxins, dosage effect, epistasis, geotropic reaction,Musa

Summary

Bunch orientation is an economically important trait in plantain (Musaspp. AAB group) and banana (Musaspp.AAA or ABB groups). Pendulous bunches are more symmetrical than subhorizontal, horizontal, or erect bunchesand are, therefore, better adapted for transportation. Erect, horizontal, subhorizontal, and pendulous bunches wereobserved in segregating populations derived from crosses between plantains and bananas and among bananas. Aproposed genetic model controlling this trait was tested in four populations. The true breeding diploid bananaline, ‘Calcutta 4’ shows a pendulous bunch and has recessive alleles at three bunch orientation loci. The diploidbanana cultivar ‘Pisang lilin’, which is heterozygous for the three loci, has a subhorizontal bunch. The triploidAAB plantain cultivar ‘Bobby Tannap’ has two simplex and one duplex loci, which also results in subhorizontalbunch orientation. The other plantain cultivar, ‘Obino l’Ewai’, which is simplex for two loci and nulliplex for thethird, has a pendulous bunch due to dosage effects at the triploid level of the recessive alleles at simplex loci. Twotetraploid hybrids (TMPx 582-4 and 1187-8) have a subhorizontal bunch because of its duplex genotype for twoloci and simplex genotype for the other locus. Bunch orientation might be an oligogenic trait regulated by theepistatic effects of at least three dominant loci.

Introduction

Orientation of plantain (Musaspp. AAB group) andbanana (Musa spp. AAA and ABB groups) bunch-es is an important economic trait. Pendulous bunchesare more symmetrical than other bunch types, mak-ing pendulous bunch orientation the most suitablefor transportation (Simmonds, 1962). Stover & Sim-monds (1987) indicated that bunch orientation inMusadepends on two factors: geotropic reaction and bunchweight.

Erect bunch orientation has been used as a defin-ing characteristic of theAustralimusaseries of culti-vars, which have 20 chromosomes. AlsoM. maclayiF. Muell., from New Guinea and Solomon Islands,and M. jackeyi from Queensland (Simmonds, 1956)have erect bunches. It has been hypothesized that thisbunch orientation results from a combination of neg-ative geotropism and a strong peduncle which allows

the fruit bunch to remain upright throughout its devel-opment.

The geotropic reaction takes place both in thepedicel and in the ovary in cultivars originated fromM. acuminataColla., donor of the A genome. Thegeotropic reaction is restricted to the pedicel in culti-vars derived fromM. balbisianaColla., donor of the Bgenome (Simmonds, 1966). Horizontal, subhorizontaland pendulous bunches are normally observed in theEumusaseries of cultivars that includeM. acumina-ta, andM. acuminata� M. balbisianahybrids. Forexample, the wild non-parthenocarpic diploid acces-sion ‘Calcutta 4’ (M. acuminatassp.burmannicoides)has a pendulous bunch while the edible diploid cul-tivar ‘Pisang lilin’ (derivative ofM. acuminatassp.malaccensis) has a subhorizontal bunch. Triploid AABFrench plantains generally have pendulous bunches.In contrast, the French plantain cultivar ‘Bobby Tan-nap’ is characterized by its subhorizontal bunch, even

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Table 1. Bunch orientation, angle between female inflorescence and pseudostem axes, axis growth, geotropic reaction, and fruit habit inMusa

Bunch orientation Angle Axis growth Geotropism Fruit habit

erect acute (< 90�) vertical upwards negative or slightly diageotropic fruits pointing in distal direction

horizontal right (90�) horizontal diageotropic fruits pointing transversally to axis

subhorizontal obtuse subhorizontal diageotropic or slightly positive fruits pointing transversally to axis

(> 90�– < 180�)

pendulous (180�) vertical downwards positive fruits pointing proximally to ground

though it has heavy bunches with many large fruits.This suggests that bunch weight has less effect thangeotropism on bunch orientation.

Barker (1960) showed that a negative geotropicreaction of the pedicel, and later of the ovary, caus-es the up-curving of the female flowers in bananas.When auxin was applied to the pedicel, the stimulatedgrowth of the treated side resulted in the bending ofthe ovary to the opposite side. Israeli (1977) found thatauxin applied to the whole fruit prevented the normalupcurving. These findings suggest that genes control-ling bunch orientation might be involved in pathwaysregulating biosynthesis or metabolism. The geotrop-ic reaction of the bunch in plantain and banana coulddepend on the different levels of auxin production.

This paper reports segregation patterns in bunchorientation in diploid and tetraploidMusa popula-tions derived from inter-diploid, triploid-diploid, andtetraploid-tetraploid crosses. Also, a proposed geneticsystem controlling bunch orientation due to geotropicreaction inMusawas tested.

Materials and methods

Three diploid (2n = 22) populations were producedby crossing ‘Calcutta 4’, as male parent, with ‘Pisanglilin’, ‘Bobby Tannap’, and ‘Obino l’Ewai’. A fourthtetraploid (2n = 44) population was produced by sib-mating the fullsib tetraploid hybrids 582-4 and 1187-8.Hybrids 582-4 and 1187-8 were derived from the cross‘Bobby Tannap’� ‘Calcutta 4’. Pollination andin vit-ro seed germination techniques are reported elsewhere(Vuylsteke et al., 1990, 1993).

Segregating populations were evaluated at the HighRainfall Station of IITA at Onne in Nigeria, WestAfrica, which is in the secondary centre of plantaindiversity. The environment at the Onne station is rep-resentative of the humid rainforest ecosystem, charac-terized by high rainfall (average of 2400 mm annually),

and highly leached, acid ultisol (pH 4.3 in 1 : 1 H2O;upper 15 cm).

Taking the pseudostem as the vertical axis, bunchorientation away from the vertical was defined as theangle between the pseudostem axis and the femalepart of the inflorescence axis (Table 1). For determina-tion of parental genotypes three classes were consid-ered: acute to right angle (or erect to horizontal bunchorientation, respectively), obtuse angle (subhorizontalbunch), and 180� angle, i.e., pendulous bunch. Obser-vations from a minimum of two plants were recordedper hybrid for subsequent analysis. The segregationdata for bunch orientation were tested using the chi-square (�2) tests of goodness of fit to expected ratioswith Yates’ correction for small sample sizes.

Results and discussion

‘Calcutta 4’, a true-breeding line (Simmonds, 1952),shows a pendulous bunch orientation. We observedsegregation for bunch orientation in the four euploidhybrid populations suggesting that pendulous bunchorientation is a recessive trait. Therefore, the diploid-diploid and triploid-diploid crosses listed in Table 2may be considered as test-crosses in disomic and tri-somic genetic systems.

Clones with erect bunches were observed in allhybrid populations. This finding was unexpected sincethis characteristic had not been previously reported inthe sectionEumusa(Simmonds, 1962), and suggestedthat erect bunch orientation might be under the controlof recessive genes interacting with specific allele com-binations at other loci. Thus, fe’i bananas (sectionAus-tralimusa) through natural selection could have fixedsimilar epistatic gene combinations. The recovery ofall types of bunch orientation in test-crosses also mightindicate that there are fewer differences between thefe’i bananas and the cultivars of the sectionEumusathan previously suggested. Simmonds (1962) inferredthat both groups evolved independently but parallel.

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Table 2. Segregation ratios for bunch orientation in plantain-derived diploid hybrids with the common male parent‘Calcutta 4’

Female parent Bunch orientation (angle between axes) �2 testz

0–90� > 90� to < 180� 180�

erect horizontal sub-horizontal pendulous

‘Pisang lilin’y 1 17 23 2 1.76 nsv

‘Bobby Tannap’x 9 16 23 8 1.16 nsu

Obino l’Ewai’x 2 6 6 8 4.35 nst

582-4� 1187-8w 4 6 39 2 2.40 nss

z Chi square tests adjusted by Yates’ correction for small sample sizes based on three angle classes.y Diploid banana.x Triploid plantain.w Sib mating between tetraploid hybrids derived from the cross ‘Bobby Tannap’� ‘Calcutta 4’.v Fit a disomic test cross ratio 3 erect/horizontal: 4 subhorizontal: 1 pendulous for a heterozygous parent for threeloci.u Fit a trisomic test cross segregation ratio 10 erect/horizontal: 13 subhorizontal: 4 pendulous.t Fit a trisomic test cross segregation ratio 4 erect/horizontal: 1 subhorizontal: 4 pendulous for a simplex parentfor two loci and nulliplex for the other.s Fit a tetrasomic segregation ratio 6 erect/horizontal: 13.7 subhorizontal: 1 pendulous.

A genetic model to explain bunch orientation inbanana consisting of at least three independent lociwas earlier proposed (Ortiz, 1995). When additiveeffects of dominant loci (d) add up to two or three,the clone shows an obtuse angle between the femaleinflorescence and the pseudostem axes, whereas it hasan acute to right angle whend = 1. Pendulous bunchesoccur due to the lack of dominant genes, i.e.,d = 0.The model assumes that erect bunch orientation resultsfrom the action of epistatic/modifier genes interactingwith one dominant locus with additive effects, i.e., a‘horizontal’ genetic background (d = 1).

This model was first tested in a segregating pop-ulation derived from ‘Pisang lilin’ (subhorizontalbunch)� ‘Calcutta 4’ (pendulousbunch). Results werepooled for three classes based on the angle betweenthe pseudostem and the female inflorescence axes: lessthan or equal to 90�, more than 90� but less than 180�,and 180�. The�2 test was not significant, which indi-cated that the observed segregation fit the expectedratio assuming that bunch orientation is an oligogenictrait under the genetic control of additive effects ofthree independent dominant loci.

Different patterns of segregation for bunch orien-tation (Table 2) were observed between the two plan-tain derived populations. This was expected becausethe plantain parents have different bunch orientations.‘Bobby Tannap’ has a subhorizontal bunch while ‘Obi-no l’Ewai’ has a pendulous bunch. Differences in seg-regation patterns reflect the different genotypes of theplantain parents. We assume that ‘Bobby Tannap’ has

more dominant alleles than ‘Obino l’Ewai’ at its bunchorientation loci.

This genetic model for bunch orientation was alsotested in the sibmating tetraploid generation. Thehybrid parents (TMPx 582-4 and 1187-8) have a sub-horizontal bunch because they have two duplex lociand a simplex genotype for the other locus. The non-significant�2 test supported the genetic model derivedfrom the analysis of diploid offspring obtained fromdiploid-diploid and triploid-diploid crosses. However,further investigation will be required to determine withgreater confidence the validity of the model proposedfor the control of bunch orientation inMusahybrids.

Potential genotypes based on segregation data fromtest crosses and phenotypic appearance of both parentsare shown in Table 3. Segregation ratios suggest that‘Bobby Tannap’ is simplex for two loci and duplexfor the third locus, while ‘Obino l’Ewai’ is simplex attwo loci and nulliplex at the third locus. The effectsof dominant alleles at simplex loci in ‘Obino l’Ewai’could have been nullified by the dosage effect (Stern,1929) of recessive counterparts at each locus. Thisresulted in pendulous bunch orientation.

Dosage effects are often observed in genetic sys-tems of triploid and tetraploidMusa (Ortiz & Vuyl-steke, 1994; Vakili, 1968). Epistatic interactionsamong additive effects of independent dominant lociare expected in vegetatively propagated species (Crow& Kimura, 1965). This genetic system might haveevolved as a consequence of a series of independent

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Table 3. Genetic model of bunch orientation in diploid, triploid and tetraploidMusa

Genotype Additive effects (d) ofBoi loci Bunch orientation

Diploid level:

Bo1-Bo2-Bo3-z 3 sub-horizontal

Bo1-Bo2-bo3 bo3 2 sub-horizontal

Bo1-bo2 bo2 Bo3- 2 sub-horizontal

bo1 bo1 Bo2-Bo3- 2 sub-horizontal

Bo1-bo2 bo2 bo3 bo3 1 horizontal

bo1 bo1 Bo2-bo3 bo3 1 horizontal

bo1 bo1 bo2 bo2 Bo3- 1 horizontal

bo1bo1bo2bo2bo3bo3y 0 pendulous

Triploid level:

Bo1Bo1bo1Bo2bo2bo2Bo3bo3box3 2 sub-horizontal

Bo1bo1bo1Bo2bo2bo2 bo3bo3bo3w 0 pendulous

Tetraploid level:

Bo21 bo2

1 Bo22 bo2

2 Bo3 bo33v 2 sub-horizontal

z ‘Pisang lilin’.y ‘Calcutta 4’.x ‘Bobby Tannap’, (2 copies ofBo1, and nil effects ofBo2 andBo3 in respective simplex locidue to dosage effects).w ‘Obino l’Ewai’, (nil effects ofBo1 andBo2 in respective simplex loci due to dosage effects).v 1187–8 and 582–4, (2 copies ofBo1 andBo2 in duplex loci, and nil effects ofBo3 in respectivesimplex locus due to dosage effects).

mutational events which accumulated during the longhistory ofMusacultivation (Simmonds, 1962).

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