Cytogenetics and reproductive biology of mangroves in Rhizophoraceae

Abstract

Three mangrove species, Bruguiera gymnorrhiza (L.) Lam., Rhizophora samoensis (Hochr.) Salvoza, Rhizophora stylosa Griff. and a putative hybrid Rhizophora ×selala (Salvoza) between the latter two species were studied for their cytogenetics and reproductive biology. Although chromosome numbers in all four taxa were confirmed as 2n = 36, the size of the chromosomes was slightly larger in genus Bruguiera. Regular chromosome pairing was observed in the three species; however, in the hybrid meiosis was very irregular. Pollen viability in all three species was very high (>80%); however, in the hybrid pollen viability was <10%. This could be due to non-homology in the chromosomes of the two parental species and irregular meiosis in the hybrid. Due to highly irregular meiosis in the hybrid and uneven distribution of chromosomes to the two poles, most of the pollen was non-viable. All three species were fully self-compatible. Between B. gymnorrhiza and the two Rhizophora species, pollen in crosses and reciprocal crosses failed to germinate, indicating cross-incompatibility between these two genera. Partial cross-compatibility was recorded between R. samoensis and R. stylosa. In many crosses, pollen tubes were observed growing freely up to the base of the style. This also accords with occasional crossing of these species in nature. However, due to only partial homology in the chromosomes of these two species, the hybrids produced are always sterile. Similarly, cross-compatibility between the hybrid and its two parental species was also partial. This could be attributed to allelic differences between the two species and their hybrid and also to physiological and/or chemical hindrance of the pollen-tube growth through the stylar tissue.