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National Tropical Botanical Garden, 3530 Papalina Road, Kalaheo, Hawaii 96741 USA
Received for publication December 21, 1999. Accepted for publication June 13, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Artocarpus altilis • Artocarpus camansi • Artocarpus mariannensis • breadfruit • chromosome number • pollen stainability • Moraceae • Pacific Islands
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Ragone, 1995, 1997
). Artocarpus altilis (Park.) Fosberg [synonyms A. communis J. R. & G. Forster, A. incisus (Thunb.) L. f.] is the most widely distributed and exhibits great variability. This species includes both seedless and seeded forms with seeded types most common in the western South Pacific. Seedless cultivars are most common in Micronesia and the eastern islands of Polynesia. Artocarpus mariannensis Trécul is always seeded and grows wild on the uplifted rock islands of Palau and the limestone ridges of the Mariana Islands. A few cultivated trees can be found growing throughout the islands of Micronesia, especially on coral atolls. It is not found in Melanesia or Polynesia other than the islands of Tokelau and Tuvalu. Fosberg (1960)
suggested that introgression between A. altilis and A. mariannensis may have occurred in Micronesia, and there are many cultivars that have characteristics of both species. A third species, A. camansi Blanco, is indigenous to the Philippines and probably New Guinea and the Moluccas. It is also found in cultivation throughout southeast Asia. This seeded species, commonly known as breadnut, has been introduced to many areas of the tropics where it is now widespread, especially in the Caribbean, parts of Central and South America, and coastal West Africa. It is rarely seen in the Pacific with the exception of a few trees in Hawaii, Samoa, Tahiti, and the Marquesas that were introduced in the past 35 yr, usually by immigrants from the Philippines. A few cultivated trees are found in Kolonia, Pohnpei, and a single tree was recently observed in a yard in Koror, Palau; otherwise it is not found in Micronesia.
Breadfruit trees are monoecious and normally cross-pollinated, with the small, powdery pollen grains spread by the wind (Jarrett, 1959
; Brantjes, 1981
). The fruit is a highly specialized syncarp (Jarrett, 1976
). The perianths of individual flowers are fused together except at the base, forming a cavity that contains the true fruit and its enclosed seed (Reeve, 1974
). As the fruit develops, this area of fusion grows vigorously and becomes fleshy at maturity, forming the edible portion of the syncarp.
Seedless cultivars may lack developed ovules or may have numerous minute abortive ovules surrounding the core of the syncarp. Few-seeded cultivars usually have one or several normal or aborted seeds. Since many cultivars of breadfruit are seedless, it has been inferred that fruit development is due to parthenocarpy (Barrau, 1976
), and Hasan and Razak (1992)
showed that the fruits of breadfruit develop normally without pollination.
Little cytological work has been done on the genus Artocarpus and even less on the many cultivars of Pacific Island breadfruit. Counts of 2n = 56 have been reported for A. chaplasha Roxb., A. elastica Reinw., A. gomezianus Wall. ex Trécul, A. heterophyllus Lam., A. integer (Thunb.) Merr., and A. lakoocha Roxb. (Habib, 1972
; Hans, 1972
; Mehra and Gill, 1974
; Chen, 1993
; Oginuma and Tobe, 1995
). Data available for seeded breadfruit basically indicate diploidy with 2n = 2x = 56; seedless cultivars are commonly triploid with 2n = 3x = 84 (Jarrett, 1959
; Barrau, 1976
). These numbers are based on counts by Janaki-Ammal (1955)
of 2n = 56 for a seeded A. communis and counts by Nishiyama and Kondo (1942)
of 2n = 54 for seeded and 2n = 81 for seedless A. communis. The latter authors acknowledged that their counts were suspect because of the poor preparations of their material.
The purpose of this study was to determine the chromosome numbers of A. altilis, A. mariannensis, and A. camansi and to clarify the basis of seed abortion and sterility in breadfruit.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Chromosome counts were based on meristematic cells obtained from root tips from three sources: germinated seeds, roots growing from 15 cm long sections of mature roots, or from airlayers made on the stems of potted plants 
2 cm in diameter and <1 m in height. Roots from all three sources were prepared in the same manner. Root tips of 3–4 mm length were excised and pretreated in a saturated solution of paradichlorobenzene (PDB) at 20°C for 2 h and fixed in Carnoy's fluid (3:1 of 95% ethanol and glacial acetic acid) for 24 h at 37°C before hydrolysis in 1 mol/L HCl for 7 min at 60°C. The root tips were stained in Feulgens solution for 1.5 h at room temperature, and slide preparations were made by squashing root meristems in a drop of 2% acetocarmine stain and Hoyer's solution. Chromosome numbers were determined from cells at mitotic metaphase using a Zeiss phase contrast microscope. Documentation was made with camera lucida drawings, and photomicrographs were taken with Kodak Technical Pan film and Ektachrome slide film at 1000x magnification.
Pollen fertility was assessed by observing the percentage of pollen grains that stained uniformly with acetocarmine. At least 500 pollen grains per accession were scored.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The report of 2n = 54 by Nishiyama and Kondo (1942)
for a seeded breadfruit from the island of Rota in the Northern Mariana Islands was probably erroneous. They identified their specimen as A. communis, but since seeded forms of this species are not known to occur in the Mariana Islands, the count was probably based on A. mariannensis. The chromosome number for A. camansi (accessions 282 and 531) is 2n = 56 (Ragone, 1991, 1997
).
Twelve seeded accessions of A. altilis were verified as diploids (2n 
56). Two accessions (256, 453) with fruits that typically yield only one to several viable seeds were also diploids. Diploid counts were also obtained for three seeded accessions from Tokelau and Micronesia (041, 301, and 363) and seven seedless Micronesian accessions (303, 322, 331, 333, 365, 373, and 375). Based on morphological characters and isozyme phenotypes (Ragone, 1991
), these 10 accessions are putative crosses between A. mariannensis and A. altilis.
Chromosome counts of 2n 84 were obtained for 20 accessions. These were all seedless with the exception of accession 379 from Pohnpei which has fruits that rarely contain one or two seeds. This cultivar, and two triploid seedless Micronesian cultivars (287 and 290), are also putative crosses between A. altilis and A. mariannensis.
The degree of pollen stainability for seeded, few-seeded, and seedless accessions is shown in Table 2. These initial studies show that the degree of seediness in breadfruit cultivars is correlated with pollen stainability. Triploid cultivars have the lowest pollen stainability, averaging from 7 to 16%, and the pollen grains are typically malformed, clumped, and poorly stained. These facts were previously noted by Tri Sunarto (1981)
, who showed that a seeded form had the highest pollen grain stainability (99%), while a few-seeded form had medium stainability (45%), and a seedless form had low stainability (6%).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Barrau, 1976
; Simmonds, 1979
), and this seems to be the case for those cultivars with 2n 84. Jarrett (1959)
also suggested that the failure of breadfruit to set seed may be due to genetic factors other than polyploidy. Since breadfruit trees are propagated by vegetative means, each cultivar is a clone within which the primary source of variation is presumably somatic mutation. Repeated vegetative propagation of breadfruit clones permits the accumulation of somatic mutations affecting reproductive fertility, which may be deleterious in nature, but which are maintained and perpetuated as curiosities or useful variants by human selection. Reduced seed number in some diploid cultivars is probably a byproduct of the practice of clonally propagating these plants using root shoots or sections of roots. Pollen sterility is also a factor contributing to reduced fertility and seed production in some diploid cultivars. In areas such as eastern Polynesia the majority of cultivars and trees are seedless triploids, which produce very little viable pollen. In Tahiti, for instance, breadfruit cultivars with seeds are so exceptional that the name for the only known seed-producing cultivar is ‘Huero’ (256), which means "with a seed." ‘Huero’ only occasionally produces one to several seeds. The proximity of seeded cultivars, which generally produce abundant pollen, results in an increased number of seeds in typically few-seeded cultivars. This has been observed in the NTBG breadfruit collection at Kahanu Garden.
Seedless diploid cultivars are also found in Micronesia. All of these appear to be interspecific hybrids between A. mariannensis and A. altilis, based primarily on shared morphological characters. Seven of the 11 diploid putative hybrids were seedless or had reduced fertility. The hybrid nature of Micronesian diploids may be largely responsible for their sterility, as with many other interspecific hybrids in which insufficient homology between genomes results in meiotic abnormalities, embryo lethality, or disruption of normal embryo/endosperm relations (Stebbins, 1971
; Simmonds, 1979
).
In Micronesia, where A. mariannensis produces abundant pollen, some of the hybrids may derive from crosses between diploid A. mariannensis and triploid A. altilis clones, which have overlapping distributions (Fosberg, 1960
). The hybrid progeny of such crosses could have additional sterility problems resulting from aneuploidy or sterility mutations derived from the triploid A. altilis parent.
Regardless of the cause(s) of sterility in breadfruit, the development of few-seeded and seedless fruits had significant benefits for Pacific Islanders who relied on breadfruit as a staple crop. The development of fruits with reduced seed numbers yielded a greater proportion of edible fleshy tissue and resulted in a shift from using this species as a nut crop (breadnut) in western Melanesia to a starch crop (breadfruit) eastwards. Since breadfruit is a seasonal crop typically available for just a few months of the year, methods had to be developed to deal with and use seasonal surpluses to provide food during the annual and often extended periods of scarcity. The method developed was that of fermentation and storage in pits. The importance of fermented breadfruit, especially in Samoa, Tonga, the Marquesas, Society Islands, and Micronesia, was a critical element in a preference for seedless cultivars that drove selection and perpetuation of seedless cultivars.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Brantjes, N. B. M. 1981 Nectar and pollination of breadfruit, Artocarpus altilis (Moraceae). Acta Botanica Neerlandia 30: 345–352
Chen, R.-Y. [ed.]. 1993 Chromosome atlas of Chinese principal economic plants. I. Chromosome atlas of Chinese fruit trees and their wild relatives, 502–503. International Academic Publishers, Beijing, China
Fosberg, F. R. 1960 Introgression in Artocarpus in Micronesia. Brittonia 12: 101–113[CrossRef]
Habib, A. F. 1972 Cytology of jack—Artocarpus heterophyllus Lam. Mysore Journal of Agricultural Sciences 6: 200–202
Hans, A. S. 1972 Cytomorphology of arborescent Moraceae. Journal of the Arnold Arboretum 53: 215–226
Hasan, S. M. Z., and A. R. Razak. 1992 Parthenocarpy in seedless breadfruit (Arthocarpus incircus (Thunb.) L.). Acta Horticulturae 321: 648–652
Janaki-ammal, E. K. 1955 In C. D. Darlington and A. P. Wylie [eds.], The chromosome atlas of flowering plants, 184. George Allen and Unwin, London, UK
Jarrett, F. M. 1959 Studies in Artocarpus and allied genera. III. A revision of Artocarpus subgenus Artocarpus. Journal of the Arnold Arboretum 40: 113–155, 40: 298–326
———. 1976 The syncarp of Artocarpus—a unique biological phenomenon. Gardens' Bulletin Singapore 29: 35–39
Mehra, P. N., and B. S. Gill. 1974 Cytological studies in Ulmaceae, Moraceae, and Urticaceae. Journal of the Arnold Arboretum 55: 663–677
Nishiyama, I., and N. Kondo. 1942 Chromosome studies in tropical plants. Report of the Kihara Institute for Biological Research 1: 26–28
Oginuma, K., and H. Tobe. 1995 Karyomorphology of some Moraceae and Cecropiaceae (Urticales). Journal of Plant Research 108: 313–326[CrossRef][ISI]
Parham, J. W. 1966 Coconut and breadfruit surveys in the South Pacific. South Pacific Commission Technical Information Paper 1. Noumea, New Caledonia
Ragone, D. 1991 Collection, establishment, and evaluation of a germplasm collection of Pacific Island breadfruit. Ph.D. dissertation, University of Hawaii, Honolulu, Hawaii, USA
———. 1995 Description of Pacific Island breadfruit cultivars. Acta Horticulturae 413: 93–98
———. 1997 Breadfruit. Artocarpus altilis (Parkinson) Fosberg. Promoting the conservation and use of underutilized and neglected crops. 10. Institute of Plant Genetics and Crop Plant Research, Gatersleben, Germany and International Plant Genetic Resources Institute, Rome, Italy
Reeve, R. M. 1974 Histological structure and commercial dehydration potential of the breadfruit. Economic Botany 28: 82–96[ISI]
Simmonds, N. W. 1979 Principles of crop improvement. Longman, London, UK
Stebbins, G. L. 1971 Chromosome evolution in higher plants. Edward Arnold, London, UK
Tri Sunarto, A. 1981 Fertility test of Artocarpus altilis pollen. Berita Biologi 2: 118
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