Cytological Studies of Some Aranda (Arachnis×Vanda) Hybrids (Orchidaceae)

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Observations on the meiotic behaviour of the Arandas have confirmed cytologically the hybrid nature of the Arandas, intergeneric hybrids between the Arachnis and Vanda forms. It is evident that the parental species of the Arandas from two widely different genera with varied differences morphologically, are not too closely related. A general trend of poor homology of parental chromosomes, high frequency of occurrence of meiotic irregularities and increased sterility was observed in the Arandas investigated. Multiple spindles, though of low occurrence may cause polyspory and hereby result in inviable gametes with variable chromosome numbers. Polyploids arising from the Arandas could possibly occur through functional diploid gamets and diads which have been observed, though these are of low frequency. Meiotic analysis of some species/hybrids of the parental Arachnis and Vanda genera confirms the regular meiotic behaviour and higher fertility within each genus. It is suggested that if doubling the chromosome number in the Arandas through, for example, colchicine treatment, is found to restore their fertility, then this would be the line of research most desirable in advancing a breeding programme for these orchids.