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Summer Scholarships 2003-2004

Phylogenetic relationships within the genus Citrus and related genera of the tribe Citreae (Rutaceae: Aurantioideae).

Supervisor: Randy Bayer


Citrus is one of the largest horticultural industries in Australia, and the breeding of new, high quality Citrus cultivars for the Australian environment depends on reliable information about the relationship of species in genera within the tribe Citreae. Wild members of Australasian Citreae may be expected to possess genes that provide a number of beneficial traits that are adapted to the Australian climate and therefore useful in Citrus breeding programs. It is therefore important to understand the internal relationships among the different species of the tribe for advancing breeding techniques and developing better conservation strategies.

The subtribe Citrineae (tribe Citreae) consists of about 21 genera and 114 species of trees and shrubs which are almost exclusively native to China and the South Pacific region, including Australia. The species of this group of plants are unique in bearing orange- or lemon-shaped fruits with pulp-vesicles filling all the space in the segments of the fruit not occupied by seeds. These pulp-vesicles are the basis for the most important fruit industry in warm countries of the world because, in species and hybrids of the genus Citrus, they contain juice of great commercial importance. Closely related to members of the subtribe Citrineae is the other subtribe of the tribe Citreae, the Triphasiinae, which contains eight genera and 46 species found in southeast Asia and Oceania, including one native Australian species.

The genus Citrus itself has been variously described as consisting of from 1 to 162 species. The most widely accepted classification systems today are those of Swingle (1946) and Tanaka (1977) who recognized 16 and 162 species, respectively. Closely related to the genus Citrus are several genera, which were recognized by Swingle. These include the genera Clymenia, Eremocitrus, Fortunella, Microcitrus, and Poncirus. Recent work has questioned the distinctness of these genera and some botanists now include them within an enlarged concept of the genus Citrus.

Relationships among species and genera of the group are complicated by several factors such as a high frequency of bud mutation, a long history of cultivation and wide cross-compatibility among species. Moreover, many species have some degree of apomictic seed production which tend to reduce variability within species leading to a proliferation of named species; so-called microspecies. Molecular genetic markers offer a way to solve many of the long standing problems in the classification and breeding of Citrus. The overall goal of this study is to provide a molecular-based evolutionary tree of relationships of members of the tribe Citreae based on nuclear DNA sequences.

Research Project

The summer student project will focus on determining relationships of the Australian members of the Citreae to extra-Australian members of the tribe. The student will be actively involved in both the limited fieldwork at an agricultural research station and the major laboratory component of the project. This will be an excellent opportunity for the successful student to learn field, glasshouse, and lab techniques used in molecular systematics and plant breeding. The student will be instructed in performing the three relatively simple molecular techniques (extraction, amplification, and cycle sequencing of DNA) and in data analysis techniques. He/she will participate in all facets of the work from the collecting of the material in the field, extraction of DNAs, DNA amplification to produce sequencing template, cycle sequencing, sequence alignment, cladistic analysis, and publication of results in an appropriate scientific journal.


Updated 4 August, 2003 by webmaster (cpbr-info@anbg.gov.au)