Genetic variability and ploidy level in species of Prosopis (Leguminosae, Mimosoideae) - Sociedad Argentina de Botánica

ISSN373-580X

Bol. Soc.Argent.Bot. 32 (3-4):217-225. 1997

GENETIC

VARIABILITY AND

PLOIDY

LEVEL IN

SPECIES OF

PROSOPIS

(.

LEGUMINOSAE

,

MIMOSOIDEAE)

PorB. O. SAIDMAN’, J. C. VILARDI’, S. MONTOYA,L. POGGIO'

Summary Geneticvariability andploidy levelin species of Prosopis (Leguminosae,Mimosoideae).Poly¬ ploidy is an uncommon phenomenon inthegenusProsopis, wheretheonly recorded cases are limitedtosome tetraploid(2n= 56;X= 14)varietiesof P.juliflora from Haiti,Colombiaand Venezuela. P. juliflora belongs to SectionAlgarobiawhichischaracterizedby lowgenetic differentiationamongthespeciesso farstudied isoenzymatically with heterozygosity(H) estimatesvarying from0.13 to 0.20and P values ranging from30 to 50%.The low incidence of interspecific genetic differentiation,lack of vegetative reproduction, and high hybrid fertilityarethecausesfor the lowincidence ofpolyploidy in thissection.Inthe present work sevenisoenzymatic systems (ADH,EST, GOT,SOD,PRX and6PGD)were characterized intwoColombian populations of P. julifloraand compared with three Argentinian populations belongingto twospecies, P. ruscifolia and P. caldenia.Ploidylevelswere also determined, 4xforP. julifloraand2x forthe other twospecies.The 20 studied lociwere homologous In all species. Some duplicated loci(Got-1 and Got-2)were observed inP.juliflorawith fixed heterozygosity.Thevariability¡nP./u//f/orapopulationswas similar to or lower thanThatof diploid species. Cytophotometric studies revealed that theDNAcontentpergenome in P. juliflora Is lower than inthediploid species.Thesignificance ofthe resultsof isozymalandcytologicalstudieswith respect to hypotheses on the originofpolyploidy is discussed.

Resumen Variabilidadgenéticayniveldeploidiaen especies de Prosopis(Leguminosae, Mimosoideae). La poliploidíaes unfenómeno poco frecuenteen elgéneroProsopis,en el cual losúnicoscasosregistrados están limitados a algunas variedades tetraploides(2n=56;X=14) deP.julifloradeHaití, Colombiay Venezuela. P.juliflora pertenecea la Sección Algarobiala cualse caracteriza porlabaja diferenciacióngenéticaentrelas especieshastaahora estudiadasisóenzimáticamente.'conestimaciones de la heterocigosis que varíanentre 0,13y 0,20y valores de Pentreun30yuñ50%.La baja Incidencia de diferenciación genética interespecífica, la falta de reproducción vegetativa, yla altafertilidadde loshíbridos son lascausasde la baja incidencia de lapoliploidíaen estasección. En elpresentetrabajo sietesistemasisoenzimáticos(ADH,EST, GOT, SOD, PRX y6PGD)fueroncaracterizadosen dospoblacionescolombianas deP.julifloray comparadas con tres poblaciones argentinas pertenecientesa dosespecies,PruscifollayP.caldenia.Sedeterminarontambiénlos nivelesdeploldía, 4x paraP. juliflora y 2x para lasotrasdosespecies.Los20loci estudiados fueron homólogos en todas las especies. Se observaron algunos loci duplicados(Got-1yGot-2) enP. julifloraconheterocigosis fijada.La variabilidadenlas poblaciones de P. juliflora fue similar o menor que en las especies diploides. Los estudios citofotométricos revelaron que el contenido de ADN por genoma fuemenor enP. julifloraqueenlas especies diploides. Sediscute elsignificadosde los resultados isoenzimáticos y citológicos en relación con la hipótesissobreelorigen delapoliplodía.

INTRODUCTION their valuableheterotic and homeostaticproperties

arelostthroughthesubsequentsexual generations. Theonlywaytopreservethese characteristics isby amphyploid geneticsystems which combinehybrid

genotypes and effectivemating ability.

Polyploidyisaphenomenoncharacteristic of the

plant kingdom, being amphiploidy the most widespreadofits variants.Polyploidyispromoted by the combination of threemain factors (Grant, 1971): 1st)theoccurrenceof naturalhybridization,

2nd) the presence of different genome or subge¬

nomesinthehybridizing diploid species,3rd)the

occurrence of perennial growing increasing the

possibility of somatic duplicationor, in annual It is a known fact that hybrids between well

differentiatied races and species show higher viability and fisiologicalhomeostasis. However,

these hybrids are usually unable to breed

effectively. Ontheonehandthey maybe

sterile/

restrictedto asexual reproduction;and if fertile,

Depto.Cs.Biológicas,Fac.Cs. Exactas y Nat.,Tjniv.Buenos Aires. 1428Buenos Aires.

'

Member of Carrera delInvestigadorCientífico yTecnológico, ConsejoNacional deInvestigaciones Científicas y Técnicas (CONICET).

Bol.Sòc. Argent.Bot. 32 (3-4) 1997

marca Departament: Altamira, 27-XI-1981, Hunziker 10039 (SI) and COLOMBIA. Bolivar

Department: Cartagena,27-XI-1981,Hunziker10040 (SI).The

samples

consisted of 10to15mother

plants

separated

50-100mfromeach other.

plants,

autogamic reproductivesystemtoincrease

the

probability

of union of nonreduced gametes from the same

hybrid.

The lack of any of this

characteristc group makes

polyploidy

rare or

absent. There are other external polyploidy pro¬ moting factors of

secondary

importancesuchas, disturbedhabitatsand

physiological

stressfavour¬

ingenvironments.

There are also

secondary

factors imposing

restrictionsto

polyploidy.

Oneofthemdependson

nucleararchitecture:a

large chromosome/

cellsize

ratio (produced by chromosome duplication) de¬ termines a mechanical limitationto proper chro¬

mosomesegregation.Another factor is thepresence

of genes preventing failuresduringmeiosis:they

reduce the

probability

of occurrence ofnonreduced gametes, which are a first necessary step for

genomicduplication

The genusProsopis includes about 44 species

distributed in arid andsemiarid zones of North

Cytological studies

Mitoticstudies

Root tips from germinating seeds were pretreatedat18-22°Cfor 3

1/2

hours in 0.02M

8-hydroxiquinoleine

solution. The fixations were madeina3:1 mixture of absolutealcohol and gla¬

cialaceticacid for more than 12hat 4° C. After

hydrolysisin5NHC1at20-22°C for30 min.staining wascarriedoutwith Feulgen(2 h)and the material

was

squashed

in45% acetic acid.The

coverslip

was removed after freezing with

_C02

andthe material

wasdehydratedinabsolute alcohol and mounted inEuparal.

and South America, Africa and Asia. One of its sections,Algarabía,involvestreeandshrubby long living speciesunabletoreproduce vegetatively.Na¬ turalhybridizationamong itsspeciesisfrequentin zones of sympatry in disturbed environments. However,theserelatedhybridizingspecies virtually

donotdifferwith respecttochromosomalrearran¬

gements, with theexceptionof indirect evidences ofcryptic structural

heterozygosity

reducing

pollen

viability in interspecific

hybrids

(Naranjo et al. 1994).

DNAcontentdeterminations

DNAcontent wasmeasuredintelophasenuclei (2 C) oftherootapexofgerminating

seeds.

Roots of 0.5-1cmlengthwerefixed in4%formaldehydein

phosphatebuffer for 2hat4°C,thenwashed24hin distilled water and transferred to 3:1 (absolute

ethanol:acetic acid) for 1-4days.

After fixationtheroots wererinsed for 30 min. indistilledwater.Hydrolysiswascarriedoutwith

5 NHC1at20° C. Different times ofhydrolysiswere

investigatedand theoptimumperiodwasfoundto

be55min.Theywerethengiventhree washes in distilledwater for 10or15 min.,staining for 120 min. inSchiff's reagent pH 2.2 (Teoh and Rees, 1976).The materialwasthen rinsed three times in

S02

waterfor 10 min.each,keptindistilled water (5-15 min.) and squashedin 45% aceticacid. The coverslipwasremovedafterfreezing

with

_C02.

The

slides were dehydrated in absolute alcohol and mounted in Euparal. The amount of

Feulgen

staining pernucleus,expressedinarbitraryunits, wasmeasuredat

wavelength

of 570nmusingthe

scanning method in a Zeiss Universal Micros¬

pectrophotometer.

The DNA content expressed in pg was

calculatedusing Allium cepavar.AilsaCraigas a

standard (2C= 33.55pg;Bennettand Smith, 1976).

This lack of chromosomal differentiation, '_{similarlytowhathappensinotherangiospermsas} Queráis,Ceanotus,Ribes,etc.,mayhave beenoneof the maincausesoftherarityofpolyploidyinthis

group where the only known species showing

diploid

and tetraploid races is P. juliflora. Po¬

pulationsofthisspeciesfromVenezuela and Haiti (Hunzikeretal.,1977) constitutethe only

examples

of

polypolidy

sofardescribedinProsopis.

MATERIAL ANDMETHODS

Population samples

Two

samples

of Prosopis

_ruscifolia

wereobtained: ARGENTINA. Formosa Province: Patiño Depart¬ ment,National Road 95, 7-1-1972,Palacios307-313-, 321-332-335-478-550-554 (BAFC); and ARGENTI¬ NA.'Santiago del Estero Province: Avellaneda

Department, Herrera, 1-1984, Saidman 146-165 (BAFC). One

population sample

of P.caldeniawas

obtained from ARGENTINA. LaPampaProvince: SantaRosa, 19-III-1981, SaidmanandEnus-Zeiger

118-126 (BAFC). TwoColombian populationsofP. juliflora were sampled at: COLOMBIA.

Cundina-Electrophoresis

Sevenisoenzymaticsystems:esterase(EST), al¬

cohol

dehydrogenase

(ADH),

glutamate

B. O. Saidmanetai,PloidylevelinProsopis

dehydrogenase

(6PGD),

aminopeptidase

(AMP),

peroxidase

(PRX)and

superoxide

dismutase(SOD),

werestudied

by

meansofhorizontal electropho¬

resisonstarch andpolyacrylamide gels, according tothe

techniques

described in (Saidman, 1985).

Depending

onthe isoenzymaticsystemsoaked seeds (ADH) or cotyledons of seven

days

old seedlings (remaining systems) were

employed,

according

toSaidman(1985).The numberofindivi¬ duals studied per locusareindicatedinTable3.

of the Section

Algarobia previously

studied

by

Saidman,(1985)andSaidmanandVilardi (1987).In the species studiedhere 24isoenzymatic locicould be identified. Almost all of themarehomologousin

all sp’ecies. The bands

corresponding

to the

allozymes

ofGot-1and Got-2inthe

diploid

species arefixedinP.

juliflora

(Table 3).Thissuggeststhat these loci would be

duplicated

in this species,

allowing

a permanent heterozygous condition at the enzymatic level. The bands correspondingto the lociEst-3,Est-4andEst-5 stainedsolightlyin

the

population

ofAltamira ofP.juliflorathatthey were almost non detectable and their allelic Statisticalmethods

TheDNA content of the three species compared by analysisof variance (nested) ofthe 2

C values

expressed

inarbitraryunits.Meanswere

compared by

Scheffé's method(Scheffé, 1959).The

confidenceinterval for theratiobetween the DNA

was

frequencies

couldnotbe

properly

estimated.These loci were excluded from the analysis of genetic variability and from thegenetic distance between

this

population

and therest.Thesameoccuredwith the locicoding for AMP in the

population

from contentof P. julifloratothat of the othertwospecies Cartagena.

was calculated according to Bliss (1967), after

previousestimationofthe components of the total -4, is

polymorphic,

with three alleles,twoactiveand

anullone.Theremainingloci in thispopulation

Meanheterozygosities (H) and theirerrors as usuallyshow theallelemostfrequentintheother

wellasNei'sdistances and identitieswereestimated species of the section. Consequently,its genetic throughthe methodsproposedbyNei (1987). These variability, estimated by the expected mean

estimates wereobtained employing the program

__

frequencyofheterozygotes perlocus (H= 0.05) and GENIND (Vilardi, 1992).Thecluster analysisfrom the percentage ofpolymorphicloci (P= 13%) was

the genetic identity matrix was made by the very low. However, in theCartagenapopulation unweighted pair groupmethod (UPGMA) (Crisci three esterase and three peroxidase loci were polymorphicand therefore the valuesobtainedfor thispopulationweresimilartothose observed in P.

ruscifolia

and P.caldenia(Table 4).

Nei's genetic identities and distances were

estimated between allpairsofpopulations(Table 5). The similarities observed betweenconspecific Prosopis

_ruscifolia

and P. caldeniaare

diploid

with

_populations,

_{wereabout 0.97, while the identities} 2n= 28 while populations of P.juliflora studied _{betweenspeciesweresignificantlylower(p<0.05),} here are tetraploid with 2n= 56 (Fig. 1). Chro- _{ranging from0.73 to 0.83.In the corresponding}

mosomesinallspeciesareverysmall andnomajor

_;phenogram

_{(Fig. 3)'SouthAmerican}

_diploid

_species

morphological

traitscanbe described. _{are morerelatedtoeach other thanto P.juliflora,}

The DNA content (2C) was estimated in all _{though these differences were not statistically}

species(Table 1) and the resultswereanalysed by significant

ANOVA (Table 2).The ratio oftheDNAcontentof

P.juliflorawith each of the other twospecieswas _DISCUSSION

obtainedwith their confidence intervalatthe 5%

InP.juliflorafromAltamira onlyonelocus,

Amp-variance.

andLópez Armengol,1983).

RESULTS

Cytologyand DNA content

level(Table 1). Botharesignificantlylower than the

expected value(Table 1) forthe

tetraploid/diploid

arid, isolated valleys

protected

fromrains. These DNA ratio.Thismeansthat theDNAcontentper dry pockets,

surrounded

bymountains andisolated genome(Table 1) issignificant lowerinP.juliflora fromeach otherbyseveral hundredkilometers,are

than expected according to the DNA content of of post-pleistocene origin and most of these populations would be less than 10,000 years old (Solbrigand Bawa, 1975).

Allozymic

variation at four systems (MDH, ADH, EST and AMP) in three

populations

ofthis

'

Electrophoretic

_{patterns of} P. juliflora (Fig. 2) _{species,onefromColombia}and_twofrom

Venezue-werecomparedwith those observed in thespecies la, was

previously

studied

by

Solbrig and Bawa P.juliflora growsinColombiaand Venezuela in

diploid species.

Bol. Soc.Argent.Bot. 32 (3-4) 1997

h

if

_•

_*

•i

•y

_*

*

iLA

*

t

•

#*

*

*

B

A

t

*«

_*

JÉ

i

#

#

*

*•

%

*»%*

a

_•v.

#

_*

tw

%

%

•

v*

S

%¡

#

T

*

•

N

•

4

C

J

D

Fig. 1.-Metaphasecellsofroottips. P.

ruscifolia

(A) andP. caldenia(B)show 2n=28chromosomes, whileP.juliflora(C and D)has2n= 56. Bar: 10 um.

Table1.-Chrómosomenumber(2n), totalDNAcontent(2C),DNAcontentper genomefor thestudied species andratiobetweentheDNAcontentofP.julifloraand thatofthediploid

species

p.j.r (conf.int.) DNA/gen

DNA (2C)(pg)

Species 2n

0.93_+/-0.04 0.90_+/-0.05 1.53+/-0.05

(1.83-1.47) (1.93-1.51) 1.64

P.caldenia P.

_ruscifolia

P.juliflora

0.465

'_0.450

0.383 28

1.70 28

56

B. O.Sàidmanetal.,Ploidy levelinProsopis

-1

»»

—2

t

—

r

i

H

—3

f

-2*

4

_—

₂₁

J

—

31

Pr

Pr

_Rr

ADH

_6-PGD

PRX

?=<;

-i*

M

—

31

I

_14

—

.I3

f-V

_—

—

51

“\43

•

t

—

2*

Fr

Pr

_P.r

AMP

EST

GOT

SOD

Fig. 2.- Zymogramsofthe seven systemsanalyzedinP.juliflora(P.r.=individualsofP.

_ruscifolia

usedascontrol for band identification).

Table2.-Analysisofvarianceto testthe differencesin totalDNAcontentamong species

Sum of

Squares

F Source of

variation

Mean Squares

Degrees P

of Freedom

<io-9

2 107.57 54.47

19.95 215.14

55.29 43.65 314.08 species

individuals cells total

<io-9 1.97

28

0.09 441

Bol. Soc. Argent.Bot. 32 (3-4) 1997

Table 3.-Allelicfrequenciesin thepopulationsstudied

P.jul. Altami.

_P./M/.

Cartag. P.caldS.Rosa

P.rusAvella. P.rusFormosa

N N

N N

N P P P P

P 0 0.25 0 Est-21 Est-22 Est-23 0.09 0 36

98 1 20 1

0.56 70 0.46

0.78 30 0 0 0.35 0.29 0.22 0.47 36 77 ? Est-3° Est-31

0.73 77 0.73

0.77 45 0.53 ? 0.27 0.27 0.23 0.17 36 77 ? Est-4() Est-41

0 79 0.41

0 56 ? 0.83 0.59 1 1 0.76 36 Est-5° Est-51

0.77 79 ?

0.55 56 0.55 79

? 0.24 0.23 0.45 0.45 1 Got-V Got-12 Got-13

0.19 0.04 1

0.08

48

60 0 29 0

50 0.81 100 0.96

0.92 1 0 1 0 0 Got-2° Got-2l Got-22 Got-23 0 0.03 0 0 0 0.97 48

60 1 29

0.79 70 0.07 100 0

1

0.77 0.97 1

0.21

0.03 0 0

0.16 0

29 1 48

Got-31 Got-32

0.91 100 1 60 1

1 70 '₀ 0 0 0 0.09 41 0.44 Prx-V Prx-V

0.61 88 0 41

100 0.98 • ₂₂

0.9

0.56

0.02 0.39 1

0.1

0.35 41

Prx-2° Prx-V

0 22 0.59 . 88 0 41

0.35 100

0.65 1

0.65 1 0.41

Prx-3° Prx-31

0.56 88 0 41 0.31 41

100 0 •₂₂

0

0.69

0.44 1

1 1

?

Amp-V 1 100 1 100 1 99 1 42

?

Amp-V 0 100 0 100 1 99 1 42

?

Amp-31 0 100 0 100 1 99 0 42

Amp-4° Amp-41 Amp-42 Amp-43 0.41 ? 0 0 0 ?

0.23 99 0.1 42

0.12 100 0.09 100

?

0.71 0.51 0

0.70

0.49 ?

0.18 0.20 0.26

Adh-V Adh-V Adhrl23

0.66 1 1

1 1

0 ₂₈ 0 20

100 0 100 0.28’ 69

0

0 0

0 0 0.06

Adh-V 1 100 1 100 1 69 1 28 1 20

6Pgd-V 1 100 1 100 0 100 0 20 0 20

20

6Pgd-31 _{1 100 1 100 1 100 1 20 1}

6Pgd-4: 0 100 0 100 100 0 20 0 20

Sod-V 1 100 1 100 1 100 1 20 ₁ 20

Sod-V 1 100 1 100 1 100 1 20 1 20

Sod-31 _{1 100 1 100 1 100 1 20 1 20}

Sod-41 1 100 1 100 1 100 1 20 1 20

B.O.Saidman etal.,Ploidylevel inProsopis

Table 4-Variabilityestimates inthepopulationsstudied

Speciesand Populations H P(%) N loci

P.

_ruscifolia

R. Nac. 95(Formosa) P.

_ruscifolia

Avellaneda(S.Est.) P.caldenia

SantaRosa (LaPampa) P.juliflora

Altamira(Colombia)

P.juliflora

Cartagena(Colombia)

0.13

*

0.04 38± 11 21

0.13± 0.04 38±11 21

0.20± 0.05 48±10 23

0.05+ 0.03 13±5 20

0.13± 0.05 30± 10 20

Table 5.-Geneticidentities(belowdiagonal)and distances(abovediagonal) betweenpopulations.

(1975), who observed very little differentiation

despitethelargedistances between them. Further¬

more,in agreement with what wasfoundhere in Altamira,theyfoundlittle variable patternsthough unfortunatelytheploidylevelwas notdetermined in theirmaterial.

When comparing differentspeciesof the section, almost all lociarehomologousand thesamealleles

arethemostfrequentin almost allpopulations.This

situationprovides an explanation for the low incidence ofpolyploidy in the section Algarobia.

Giventhehigh interspecific similarity,asignificant

heterosis through hybridizationis not expected.

Therefore, polyploidy would not be an efficient mechanism to fix advantageous genic

combina-3

1 2 4 5

0.033 0.210 0.319 0.231 1)P.

_ruscifolia

Formosa 2)P.

_ruscifolia

Sgo.Estero 3) P.caldenia LaPampa 4)P.juliflora Altamira 5) P.juliflora Cartagena

0.967 0.192 0.279 0.205

0.811 0.825 0.288 0.202 0.727 0.756 0.750 0.024

0.794 0.815 0.817 0.976

0.72 0.80 0.88 0.96 1.04

rusF

rusS

calLP

JulA

julc

Fig.3-Phenogramrepresentative of therelationshipsamongthestudied populations.asobtained fromthegenetic identitiesthroughLJPGMA (rusF:P.

_ruscifolia

Argentina.Formosa Province: PatiñoDepartment;rusS:P.

_ruscifolia

Argentina.SantiagodelEsteroProvince:Avellaneda Department,Herrera; calLP:P.caldenia Argentina.LaPampa Province: SantaRosa;julA:Colombia. Cundinamarca Department:Altamira;julC: Colombia. Bolivar Department:

Cartagena.).

Bol. Soc.Argent.Bot. 32 (3-4)1997

Saidman and Vilardi, 1987, 1992). One of the

alternative

possible

explanations for this

high

similarity

among

Algarobia

species withinthe Cha¬

co

Biogeographic

Region (Saidman 1985;Hunziker

etal.,1986; SaidmanandVilardi,1987) isthe very

frequent

hybridization and introgression between them leading to the homogenization of allelic

frequencies.

This isnotagood

explanation

for the

identity (1= 0.73-0.82) here observedbetween P. julifloraandtheothertwospecies.These valuesare

about the

expected

for semispecies (accordingto Ayala et al. 1974) despite the fact that they are

completely

isolated both

genetically

(as a consequence of the different

ploidy

level) and

geographically.

These

hypotheses

arenotnecessarily exclusive. In bothcases, thedivergencebetween P. juliflora

andArgentineAlgarobia speciesisexpectedtohave occurred duringa

period longer

than that which has elapsed for the divergence within the latter

group.Indeed,thisdivergencemusthave occurred

after

polyp

loidization took place, and the only . difference resides inthe relative length of these

stages. But in bothcases,thegenetic variabilityhas been maintainedwithminor changes.In fact the

.

geneticdistance betweenP.

_ruscifolia

and P.caldenia

isnot

statistically

shorterthanthatbetweenanyof them andP.juliflora.

Sinceseveral facts indicate that in America the

principalcentreoforiginofProsopis speciesis Ar¬

gentina

(Burkart 1976),P.juliflorawould havetobe

recent.The fact thatitsvariabilityissimilartothat .ofspeciesexpectedtobe older again

implies

that

thisdegreeof variabilityhas been maintainedby

selection during colonization and species diver¬ sification.

.

tions. However,some

duplicated

lociareobserved

in P.

juliflora

inwhich apparently the

heterozy¬

gosity became fixedatthe molecular level (Got-1

andGot-2).

In

tetraploids

ofrecentorigin the intergenomic diversity (which would be

expressed through

peptide

complementation)is

usually higher

than the intragenomic (allelic

variability

=H),while the older

tetraploids

present a higher intragenomic

diversity

even morethansome

diploid

species.Ini’.

juliflora

intergenomic diversityisobserved

only

for

GOTloci, but the values estimated forH andPare

similaror evenlower thanthosefound sofar for

diploid

speciesin the Section Algarobia. This is

compatible

with the

hypothesis

ofa recentoriginof the

tetraploid,

which, therefore, would havenot

yet increased itsintragenomicdiversity. Thisisalso consistent with the fact thatthe

populations

from

Colombia and Venezuela of this species are less than 10000yearsold. Also in agreementwiththis

assumption's

the factthatit is notmorphologically distinguishable from diploid cytotypes of the

«samespecies».

Nevertheless, the results from the

cytopho-tometric study are discordant.Recent tetraploids are expected to conserve the sameDNA content per genome as the original diploids, while in ancientones areduction ofthisvalueusuallyoccurs as a responsetoselectiveprocesses tendingto fit

the cell cycle to the requirements of the species

(Bennett andSmith,1976).In thepresent workit was found that the diploids P.

_ruscifolia

and P. caldenia do not differ, while P. juliflora shows a

reduced amount of DNA per genome. Two

hypotheses

might be advanced to explain this

disagreement.

1) Theoriginal

diploid

fromwhich P. juliflora originated had a lower

DNA.

content than the

diploid

species here studied. Since P.juliflora is

consideredanautotetraploid,this

hypothesis

could betested

by

studying theDNAcontentofdiploid

cytotypes whicharethemostplausibleancestorsof

tetraploids.

2) The allelic frequencies and the degree of

variability aremaintainedby selection at similar

levelsinmostAlgarobia speciesas a

consequence

of

similar

adaptive

strategies. The high genomic

coadaptation,

allowinga high adaptability and

colonizing ability,would leadtoparallelevolution and conservation of high interspecific similarity through long periods.Thishypothesis agreeswith the overallhigh genetic similarityexhibitedamong

thespeciesofthis sectionstudiedsofar.Indeed, the

genetic identities among species of Section

Algarobia

ranged betweenthevaluesexpectedfor semi- or even

subspecies

(Saidman, 1985, 1993;

ACKNOWLEDGMENTS

Wewish to express our gratitude to Dr._JuanH. Hunziker and Ing. Agr. RamónA. Palacios whokindly

donated part of the materials studied here. Our thanksto

-Ing. Agr.P. Steibel and Ing. Agr. H. Troiani for the "

taxonomicidentification and assistance in collecting

materials from La PampaProvince. This work was financiatedby theConsejoNacionaldeInvestigaciones

Científicas y Técnicas (CONICET) (grants to

_Juan

H. Hunziker and LidiaPoggio),Universidad de BuenosAires (grants to Beatriz O. Saidmanand LidiaPoggio) and InternationalFoundation for Sciences (IFS)(grantstoBea¬ triz O. Saidman).Technical assistance inlaboratorywork byLie. ArianaGigenaisgreatly acknowledged.

BIBLIOGRAPHY

AYALA F.J.,M.L.. TRACEY, D. HEDGECOCK &.R.C. RICHMOND.1974. Genetic differenciationduring

B.O.Saidmanetal., Ploidylevel in Prosopis

BENNETTM.D.&_J.B.SMITH.1976.NuclearDNAamounts

_NARANJO

C.A., L. POGGIO& S.ENUS-ZEIGER.1984. PhenolChromatography,morphologyandcytogene¬ ticsin three species andnaturalhybridsofProsopis

(Leguminosae-Mimosoidae). PI. Syst. Evol. 144: 257-276.

in angiosperms. Philos.Trans.,Ser. B 274:227-274. BLISS,C.1.1967.StatisticsinBiology.StatisticalsMethods

for researchin NaturalSciencesVol. 1.MeGraw-Hill BookCompany.New York.

BURKART, A.1976.Amonographofthe genus Prosopis NEI, M.1987.Molecularevolutionarygenetics. Columbia

UniversityPress.NewYork.

SAIDMANB.0. 1985.Estudio de la variabilidad alozímicá (Leguminosae subfam.Mimosoideae)._/.Arnold Arbor.

57(3): 219-249.

CRISCI

J.V.

&M.F. LOPEZ ARMENGOL.1983.Introduc¬ ción alateóriay práctica de lataxonomía numérica.

Org.Est. Amer.WashingtonD.C.

GRANT V.1971.PlantSpeciation.Columbia University Press.

enelgéneroProsopis. Tesis de doctorado .Facultad de Cs.Exactas y Naturales.UBA.

1993.Lasisoenzimasy el estudio de lavariaciónge¬ néticay las afinidadesentreespecies deProsopis (Legu-minosae).Bol.Genét.Inst.Fitotéc. Castelar.16: 25-37. HUNZIKERJ.H., C.A.NARANJO,'R.A. PALACIOS&L.

POGGIO.1977.Chromosomalcytologyandhybridi¬

zation. In: Simpson, B.B. (ed.). MESQUITE.ITS

BIOLOGYINTWODESERTECOSYSTEMS.US/IBP. SCHEFFEH. 1959.Theanalysisofvariance.Wiley.New York.

&_J.C.VILARDI.1987.Analysisof genetic similarities amongseven species of Prosopis (Leguminosae: Mimosoidea).Theoret.Appl.Genet. 75:109-116. Series 4.Ch.3.«Patterns ofvariation».Pensylvania:

Dowden,Hutchinson andRoss,Inc. SOLBRIGO.T. &_KAMALJITS.BAWA.1975.Isozyme variationinspecies of Prosopis(Leguminosae).

J.

Arnold Arbor. 56:398-412.

TEOHS.B.&H.REES.1976.NuclearDNA amountsin

populationsofPiceaandPinusspecies.Heredity36: 123-137.

HUNZIKER J.H.,B.O.SAIDMAN, C.A. NARANJO,R.A. PALACIOS, L. POGGIO& A.BURGHARD.1986.

Hybridization andgenetic variation of Argentine speciesofProsopis.ForestEcol. Manage.165: 301-315. MONTOYA S.,B.O.SAIDMAN,J.VILARDI&C.BESSEGA.

1994.Diferenciación yflujogenéticoentreespeciesde VILARDI,

_J.C.

1992.Genind: ProgramaBasicparaestimar lasecciónAlgarobia,GeneroProsopis (Leguminosae).

ActasXXIV Congreso Argentino deGenética.La Plata, Argentina.

índices de distanciay variabilidad genéticasy sus errores apartir demuestraspequeñas. Mendeliana10: 71-74.