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Chromosomal Pairing and Pollen Viability
in Rhizophora mangle and Rhizophora stylosa Hybrids
Anand P. Tyagi
Department of Biology, University of the South Pacific, Suva, FIJI

Abstract
Two prominent mangrove species of Fiji, Rhizophora mangle, Linn. Rhizophora stylosa Griff and their puative hyubrid (R x selala) were analysed for chromosome number and pairing. Both parental species and their hybrid possess a diploid number of (2n) = 36 chromosomes. Regular 18 bivalents were observed in two species but the hybrid lacked proper chromosome pairing during meiosis. Analysis of tetrads showed normal tetrad and microspores development in parental species but very high abnormality in the hybrid. Pollen fertility determined by staining technique and pollen germination technique showed very high pollen viability in both parental species but very low pollen viability in the putative hybrid. Lack of chromosomal homology appears to be contributing to high percentage of non-viable pollen resulting in complete sterility in the putative hybrid.

Introduction

Mangroves have very special place in the lives of Pacific islanders. These trees provide wood for fire, fuel, furniture and artifact making. Their fruits (propagules) are eaten when there is shortage of food. Mangroves protect seashore from degradation and also provide sanctuary for young crustaceans. Two species of these mangroves cross in nature and produce sterile hybrids. Hybrids normally grow faster and produce better quality timber. In the present investigation two parental species and their hybrids were studied for chromosome pairing and pollen viability. This study determined the cause of sterility in hybrids, which in turn will be helpful to propagate these hybrids by making them fertile using method of chromosome doubling.
Mangroves form very important plant communities, found on protected shores mostly in the tropics and subtropics (Macnae, 1966). Mangroves grow very well in regions where there is an abundance of stable silt, which receives a mixture of nutrient laden fresh water and/or oceanic salt water. In Fiji three mangrove species in the family Rhizohoraceae are found in abundance namely; Bruguiera gymnorhiza Linn. Rhizophora mangle Linn. and Rhizophora stylosa Griff. Two of the species – Rhizophora mangle and Rhizophora stylosa cross in nature and produce sterile hybrids (Rx selala). Wherever these two species occur together, hybrid trees are also found. These hybrid trees possess intermediate characteristics between two parental species. Their leaves, buds and flower can clearly be distinguished from two parental species. The hybrids flower profusely as do the parental species but do not set propagules. Therefore, it might be assumed that the hybrids between these two species are completely sterile, most probably due to failure of chromosomal pairing gamete formation. The present study was designed to test this hypothesis using chromosome analysis, tetrad formation and pollen fertility assessment in two parental species and their putative hybrid.

Discussion
Both parental species of the putative Rhizophora mangle and Rhizophora stylosa showed regular chromosome pairing and segregation at meiosis-I. However, the putative hybrid completely lacked perfect chromosome pairing at meiosis-I. Most dividing cells of the hybrid had univalents at prophase-I and metaphase-I and lagging chromosomes during anaphase-I. Stebbins (1958) ascribed this phenomenon due to non-homology of chromosomes in two species. The two parental species are close enough to cross in nature and produce hybrids but the hybrids remain sterile due to non-homology of chromosomes. Hacker (1968) reported failure of chromosome pairing in a probable Desmodium intortum x Desmodium sandwicense hybrid. Due to failure of chromosome pairing during meiosis in hybrid between Rhizophora mangle and Rhizophora stylosa abnormal tetrads were formed. This led to shriveled microspores (Table 2) and extremely low pollen fertility in the hybrid. McKeller and Quesenberry (1992) reported low pollen fertility and very low viable seed production in synthetic hybrids between Desmodium ovalifolium x Desmodium heterocarpon crosses. However, in case of the synthetic hybrids between two Desmodium species, lack of chromosomal homology did not appear to be the reason for low hybrid seed production in controlled crosses (McKeller and Quesenberry, 1992).

McWhiter (1962-63) reported fertile and vigorous hybrids between Desmodium sandwicense and Desmodium intortum. There are numerous examples in animal and plant kingdom where two related species cross in nature and produce viable but sterile hybrids (Arnold; 1997, Howard and Berlocher; 1998). In an earlier study Tyagi and Singh (1998) also reported low pollen viability in a putative mangrove hybrid between Rhizophora stylosa and Rhizophora samoensis. In these cases back crossing and formation of an F2 population is extremely unlikely. Hybrid swarms and introgression between the species will be very low. Thus despite the cross compatibility the two parental species likely to remain distinct.

mangrove mangrove
Source :
S. Pac. J. Nat. Sci., (2002), Vol. 20, pp. 1 – 3 Section A: Featured Article

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