Pachytene chromosome morphology of Artemisia annua L.

Abstract

Artemisia annua L., a member of Compositae (Asteraceae) family, is a sexual and diploid species with 2n=2x=18 (Suzuka 1950). The leafy portions of this herb contain a potentially important antimalarial agent artemisinin (qinghaosu, QHS, arteannuin) which has been used by the Chinese to cure patients infected with Plasmodium vivax and P. falciparum and is found effective against both chloroquine-sensitive and chloroquine-resistant strains of P. falciparum, as well as against cerebral malaria (see Klayman 1985). Due to the usefulness of artemisinin and its derivatives to treat malaria clinically, A. annua has been recently introduced in India by this Institute from England (Singh et al. 1986) and is being successfully cultivated in Kashmir Valley and Northern plains of the country. The cytogenetic and breeding approaches to produce improved A. annua cultivars are also underway.To construct chromosome maps and assigning linkage groups for use in genetic and cy togenetic investigations, the individual identification of the different chromosomes of the com plement is a prerequisite. The somatic chromosomes of A. annua are relatively smaller in size and lack enough difference in morphological details. These similarities preclude identi fication and analysis of each single chromosome in the somatic complement. On the other hand, at pachytene stage of meiosis, chromosomes are highly differentiated. Homologues are paired in an extended condition and the individual bivalents are identifiable by their dis tinctive morphological features. Since, so far no information on pachytene chromosomes of A. annua is available, this investigation was initiated to define the morphology of pachytene chromosomes as a part of the programme on the genetic improvement of this species.The present paper is concerned with the morphology of the pachytene chromosomes of A. annua. Particular emphasis is placed on the identifying characteristics of each chro mosome. Materials and methodsThe plant material used in this study was original introduction of A. annua from the Royal Botanical Gardens, Kew, England. Most of the pachytene data were obtained from a single healthy plant grown in the field. The young flower buds were collected and fixed directly in a freshly prepared solution of 6 parts of absolute alcohol, 3 parts of chloroform and I part of glacial acetic acid. Ferric chloride (1g/100ml fixative) was added to intensify the staining of the chromosomes. After 24 hrs of fixation at room temperature, they were transferred to 70% alcohol and then refrigerated until examined. The anthers were squashed in 1% acetocarmine. Judicious heating of the slides, after squashing, greatly helps in spread ing of pachytene chromosomes.Phase contrast optics were used for observations and microphotography. For all meas urements and the study of morphological details, cells at mid-pachytene stage were selected from temporary slides. The morphological comparisons were made either with the help * CIMAP Publication No, 831A