Dickson, D.D.

Huettel, R.N.

Kaplan, D.T.

Horizon / Pleins textes

Chromosome number of 
Populations of Radopholus similis from North, Central, 
and South America,  Hawaii, and Indonesia ( l )  
Robin N. HUETTEL *, Don D. DICKSON * *  and David T. KAPLAN * * *  
* USDA,   ARS ,   P lan t   Pro tec t ion   In s f i t u t e ,   BARC-Wes t ,   Be l t s v i l l e ,   Md   20705 ,   USA 
* *  University of Florida, Gainesville, PL  32611 U.S.A. and 
* * *  U S D A ,   S E A ,  ARS, Horticultural Research Laboratory, 2121 Camden Rd . ,  Orlando, F L  32803  U.S.A. 
SUMMARY 
Chromosome numbers were determined in seventeen populations of Radopholus similis  from widely separated 
geographic  regions of the world,  viz. Honduras, Costa  Rica, Panama, Mexico, Ecuador,  Guatemala,  Hawaii (U.S.A.), 
Indonesia and Florida  (U.S.A.). Al1 specimens  examined  had  haploid  chromosome  numbers of either  n = 4 or n = 5, 
the  same  numbers  found,  respectively,  in  the  banana  and  citrus  race of R. similis from  Florida.  Five  populations 
had been characterized previously as possible biotypes of these races based on differences in  their  reproductive 
rates. No variation in  chromosome numbers  was  observed among  biotypes.  The  consistency  in  chromosome  numbers 
further  demonstrates  the  reliability of the  karyotype  for  distinguishing.the  banana  race  (n = 4) from the  citrus 
race  (n = 5). 
RÉSUMÉ 
Nombre  chromosomique  de  populations  de Radopholus  similis originaires  d’Amérique du Nord,  
Centrale et du  Sud  ainsi   que des Hawai i  et d’Indonésie 
Le nombre  de chromosomes a été  déterminé chez dix-sept  populations  de Radopholus  similis provenant de régions 
géographiques bien distinctes : Honduras,  Costa-Rica,  Panama, Mexique, Equateur,  Guatemala,  Hawaii (U.S.A.), 
Indonésie e t  Floride  (U.S.A.).  Pour  queIques  pays,  plusieurs  populations  ont  été  examinées.  Dans  tous le  spécimens 
observés,  le  nombre  chromosomique  était  n = 4 ou n = 5, ce qui correspond aux  nombres  haploïdes  trouvés res- 
respectivement chez les races a banane e t  (( citrus de R. similis en Floride. Cinq de ces populations avaient déjà 
été caractérisées comme biotypes  probables  de  ces  deux  races  en  raison  de différences observées dans  leur  reproduc- 
tion.  Les  résultats  obtenus  montrent qu’à l’intérieur de  chaque  biotype  il  n’existe  pas  de  variations  dans le nombre 
de chromosomes. Ceci confirme que le caryotype est un critère valable pour établir la distinction entre la race 
(( banane (n = 4) et la race (( citrus (n  = 5). 
Uniformity  in  the  number of chromosomes seems 
to be  characteristic  in al1 bisexual  nematodes species 
thus far examined (Triantaphyllou & Hirschmann, 
1980). For example,  the  chromosome  numbers  within 
three  bisexual species of Pratylenchus were  n = 5, 6, 
and 7 (Roman & Triantaphyllou, 1969). Populations 
of these Pratylenchus spp.  were  from  several  locations, 
yet  their chromosome  numbers were. consistent. 
Differences in chromosome numbers within or be- 
tween  populations of a nematode  species  would  indi- 
cate a  major genic change tha t  would  be of evolution- 
ary importance. White (1978) pointed out  tha t  90- 
95% of al1 speciation  events  have  been  accompanied 
by karyotypic changes. Such karyotypic differences 
are also of considerable taxonomic value in many 
other  groups of animals. 
The  banana  and  citrus  races of Radopholus  s imil is  
from  Florida were  found to  have a chromosome  num- 
ber of n = 4 and n = 5, respectively (Huettel & 
Dickson, 1981). This difference in chromosome  num- 
ber  between the  two  races  may  indicate  an  evolution- 
arily  significant mutational  event. It is  probable tha t  
either a chromosome  fusion or a dissociation  occurred 
in  one  race  to give  rise to  the  other.  However, 
because nematodes have either holocentric or poly- 
centric chromosomes (White, 1974 ; Fiil, Goldstein 
(l) Florida Agricultural Experiment Station Journal Series. No. 4121. Portion of a dissertation submitted by 
the senior author in partial fulfillment of the requirements for the Ph. D. degree in Nematology, University of 
Florida,  Gainesville,  FL. U.S.A.  32611. This  research  was  supported by a USDA, SEA,  ARS,  Cooperative  Agreement 
Grant, No.  58-7B30-213 and  in  part  by a grant  from  United  Brands Co., New York, N Y 10020. 
Revue  Nkmatol. 7 (2) : 113-116 (1984) 113 
R.N. Huettel, Don D.  Dickson & D.T. Kaplan 
& Moens, 1977),  the  mechanics of which  are  not well 
understood, the most probable direction of the  mu- 
tation cannot be elucidated currently. 
The literature on physiological races of R. similis 
is  complicated by  the  description of biotypes of each 
race  (Pinochet  1978,  1979 ; O’ Bannon & Ford, 1979 ; 
Tarte & Pinochet  1981).  An  “isolate”  was defined by 
Pinochet (1979) as a populat>ion of the banana race 
tha t  differs in reproductive capabilities from other 
populations of the race. O’ Bannon  and  Ford  (1979) 
described two possible biotypes of the  citrus  race as 
populations th&  can  break  resistance  in  root-stocks 
of Citrus spp. The terms “biotype”, “isolate”, and 
“race” are often used interchangeably, thus a clari- 
fication of their  definitions is necessary  before  descri- 
bing populations of R. similis. “Race” will be used 
here ta  denote a selective  difference in  host preference 
within a species of plant parasitic nematode (e.g., 
citrus race). “Biotype” will be used to denote any 
unusual  characteristic  exhibited by  a population of a 
described  race  ( .g.,  reproductive  differences or 
resistance  breaking of a resistant  variety of its  normal 
host).  Four  biotypes of the  banana  race  described by 
Pinochet  (1979)  and  one  citrus  race  biotype  described 
by O’ Bannon  and  Ford (1979)  were included  in 
this  study . 
The objectives of this study were to determine i )  
the existence of karyotypic  uniformity  among  popu- 
lations of the  banana  race  (n = 4), i i )  the  karyotype 
of populations of R. similis from other host plants 
from  both  Florida arld other  locations  worldwide,  and 
i i i)  the occurrence of karyotypic differences within 
biotypes of the  same  race. 
Materials and Methods 
Seventeen populations of R. similis from widely 
separated  geographic  regions of the world  were  main- 
tained  by  carrot disc  culture  (Huettel,  1982)  for 
cytogenetic analysis. Six populations of the banana 
race of R. similis were from collections rnaintained 
on disc culture a t  t h e  Tropical Research Division, 
United Brands Fruit Co., La Lima, Honduras. The 
populations  had been  previously  described as 
“biotypes” by Pinochet (1979) and were originally 
collected  from Musa sp.  from  the following  locations : 
Tapachula, Mexico ; La  Lima,  Honduras ;Coto,  Costa 
Rica ; Armuelles, Panama ; Changuinola, Panama ; 
and Machala,  Ecuador.  Another  population of R. 
similis from Central America was cultured from a 
shipment of quarantined Musa sp.  from  Guatemala. 
Its  exact  origin  is  unknown. One additional  popula- 
tion of the  banana  race  was  obtained  from Musa sp.  
growing in an abandoned commercial plantation in 
Dade  County,  Florida. 
114 
. Two populations of the citrus race, one of which 
was considered to  be a biotype by O’ Bannon and 
Ford  (1979),  were  obtained  €rom  the U.S.D.A. Horti- 
cultural Laboratories, Orlando, Florida. Two popu- 
lations  originating  from  infected  citrus  roots  growing 
in  Polk  County  were  obtained, also from  the  Florida 
Department of Agriculture  and  Consumer  Service 
(FDACS),  Burrowing  Nematode  Laboratory,  Lake 
Alfred,  Florida. 
Other populations used in this study were from 
quarantined  shipments of Philodendron sp. and 
Maranta sp., FDACS, Division of Plant Industry, 
Nematology  Section, Gainesville, Florida,  and Anthu- 
rium andraeanum L. and Musa sp., Department of 
Plant  Pathology,  University of Hawaii,  Honolulu, 
Oahu. An Indonesian population from black pepper 
(Piper  nigrum L.) was  examined at  Rothamsted 
Experimental St,ation, Harpenden, Herts., England. 
The  method  for  determining  chromosome  numbers 
was  modified from  Triantaphyllou  (1975)  and re- 
ported  previously  (Huettel & Dickson,  1981).  Ap- 
proximately 200 to 500 female nematodes  were 
examined  for  each  population. 
Observations 
The number of chromosomes established for each 
population examined is listed in Table 1. Al1 popu- 
lations  from  banana  had  four  chromosomes  (examples 
illustrated  in  Fig. 1). The  Indonesian  population 
from  black  pepper  had  four  chromosomes as did 
populations from the ornamentals, Philodendron sp. 
and Maranta sp. The four populations from Citrus 
spp.  and  the  one  from A. andraeanum had five 
chromosomes  (examples  illustrated in  Fig. 2). 
Discussion 
Karyotypic  uniformity  within  the  banana  nd 
citrus races of R. similis is important in clarifying 
their  taxonomic  status.  Eight  populations  that orig- 
inated  from  banana  nd  three  populations  from 
Philodendron sp., Maranta sp.,  and  black  pepper  were 
determined  to  have  four  chromosomes.  Although 
many ornamentals are known to be good hosts of 
both races (Poucher et al., 1976), both populations 
obtained  from  quarantined  ornamental  shipments 
from  outside of Florida  had  four  chromosomes.  Popu- 
lations  from Citrus spp.  from  Florida  and A. andrae- 
anum from  Hawaii  had five chromosomes. 
The Hawaiian population of R. similis from A. 
andraeanum had five chromosomes like the Florida 
citrus  race.  This is the first  report of a  five  chromo- 
some population of R. similis outside the state of 
Florida. Therefore the five chromosome populations 
Revue  Nématol. 7 (2) : 113-116  (1984) 
Chrornosome  number of populations of Radopholus  similis from North,   Central,   South  America,  Hawaii ,   Indonesia 
Fig. 1. Photographs of metaphase 1, n = 4 chro- 
mosomes of the  banana  race of Radopholus  similis 
from : A : Tapachula, Mexico, B : La  Lima,  Hondu- 
ras, C : Armuelles, Panama,  and D : Machala, 
Ecuador.  (Chromosomes darkened  tonhance 
resolution). 
Revue   N imato l .  7 (2) : 1.73-116 (1984) 
j 
Fig. 2. Photographs of the chromosomes of four 
populations of the citrus race of R. similis. A : 
Citrus,  Polk  County,  FL, oogonial metaphase,  n = 
10, B : Citrus, Polk County,  FL,  metaphase 1, 
n = 5, C : Orlando, PL, metaphase 1, n = 5, and 
D : Anthur ium,  Hawaii,  metaphase 1, n = 5. (Chro- 
mosomes darkened  to  enhance resolution.) 
115 
R.N. Huef fe l ,   Don  D.   Dickson  & D.T .   Kap lan  
are not restricted to Florida as previously assumed. 
Consequently, five chromosome races of R. s imi l i s  
may  have  evolved  with  the  four  chromosome  race  in 
another area of the world and was spread subse- 
quently  with  crops  and  ornamentals  other  than 
Citrus spp. 
Even  though  there is a difference in  chromosomal 
number between the  citrus  and  banana  races,  no  such 
differences  were  found within  the  races.  The  biotypes 
of either  race  examined  had  the  chromosomal  number 
expected for that  race.  Therefore,  chromosomal 
numbers  are  not  implicated  in  the  way  each  biotype 
responds physiologically. This is not  to  say  that   the 
reported differences among the biotypes within R. 
s imi l i s  populat,ions  are  not  genetically  induced.  Alpha 
karyology (counts of number of chromosomes and 
their  approximate size)  does not allow  one to  draw con- 
clusions about  other  possible  genetic  mechanisms 
tha t  could bring  about  the  differentiation of biotypes. 
The minute size of the chromosomes also prevents 
observation of any  types of chromosomal  rearrange- 
ments that may exist among biotypes of the same 
race. 
TABLE 1 
Origin,  host, and haploid  chromosome  number 
of seventeem  populations of Radopholus  similis  
Haploid 
Number  
Origin  Host  Chromosome 
Tapachula, Mexico 
Guatemala 
La Lima, Honduras 
Coto, Costa Rica 
Armuelles, Panama 
Chanquinola, Panama 
Machala, Ecuador 
Dade County, Florida 
Unknown 
Unknown 
Hawaii 
Indonesia 
Florida 
Florida 
Polk  County,  Florida 
Polk  County,  Florida 
Hawaii 
banana 
banana 
banana 
banana 
banana 
banana 
banana 
banana 
Philodendron sp. 
Maranta  sp. 
banana 
black  pepper 
citrus 
citrus 
citrus 
citrus 
A n t h u r i u m  
4 
4 
4 
4 
4 
4 
4 
4 
4 
4 
4 
4 
5 
5 
5 
5 
5 
Karyotypic  uniformity  is  expected  in  sexually 
reproducing  diploid species (White,  1978). Differences 
in chromosome number  indicate  that  the  two  races 
are  distinct  species  (White,  1978).  The use of cytogen- 
etic techniques is a quiclr and reliable method for 
identification of populations of R. similis. The  tech- 
nique may be of value also in understanding phy- 
logenetic  relationships  between  races. 
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116 Revue  Nématol .  7 ( 2 )  : 113-116 (1984) 


Chromosome number of populations of Radopholus similis from north, central and south America, Hawaii, and Indonesia

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