Developmental genetics of allometric variation between species

As living organisms grow, they also change shape. The human head, for instance, grows more slowly than the rest of the body, and so makes up less of the total size of an adult than a baby. Similar changes in shape and size have been important in the evolution of new species and are important in the breeding of crops and livestock (e.g. plants with bigger fruits). Although the importance of changes in both size and shape was recognised over 80 years ago, when the phenomenon was named allometry (from the Greek allo ` different¿ and metron `measure¿), its causes are still mostly unknown. We have used plant leaves and petals as examples of growing shapes to understand how allometry is controlled. We hybridised two snapdragon (Antirrhinum) species with different sized leaves and petals. By comparing the size of these organs in each of the offspring with the genes it had inherited, we were able to locate the genes responsible for differences in organ size. These genes also affected shape (i.e. controlled allometry). We will use this finding along with new techniques and plant varieties that we have developed, to make the first integrated study of the causes of natural variation in allometry. We will attempt to discover how leaves and petals change shape as they grow and how allometry genes affect these changes. This will involve approaches similar to medical CAT scanning and time-lapse photography to image growing organs. Because leaves and particularly petals are not completely flat, we will develop computer software to navigate and record 3D shapes from these images. We will also examine whether the same genes are responsible for differences in leaves and petals between other snapdragon species and how the genes act together to control the final shape and size of organs. Because a plant normally produces leaves with different shapes and sizes from the bottom of the stem to the top (a phenomenon known as heteroblasty), one intriguing question is whether heteroblasty and allometry are related (e.g. are they controlled by similar genes). Finally, we will try to identify the DNA sequences of at least two of the allometry genes. Later this will allow us to study how the proteins produced by these genes influence growth and to examine how mutations in the genes have allowed evolution of leaves and petals with different shapes and sizes. Joint with BB/D522089/1 and BB/D522438/1

Much of the variation between species and varieties involved correlated differences in shape and size (allometric variation). Despite its importance, little is known of the genetic or developmental basis of such allometric differences. We will build on our work on shape and size variation between closely related species of Antirrhinum, using complementary genetic, developmental and computational approaches to analyse the processes underlying natural allometric variation. (1) Plants carrying individual allometry genes in a uniform background will be used to analyse how organ shape and size changes with development in different genotypes. Computational methods will be developed to describe such changes quantitatively in both 2D and 3D. The observed allometric changes will be related to regional differences in growth, determined by clonal analysis and tracking of landmarks, and to the cell proliferation and expansion involved in this growth. (2) To examine allometric variation in a wider context, we will analyse F2 populations from further species crosses, and introduce alleles carried by different species at key allometric loci into a uniform background to compare their effects. (3) Different combinations of allometry genes, in hybrid populations and in a uniform genetic background, will be used to assess how they interact to determine shape and size. (4) The relationship between heteroblasty (variation in leaf shape and size with position on the plant) and allometric variation between genotypes will be examined using a combination of genetic, computational and developmental methods. (5) Key allometry loci will be fine-mapped and transposon-mutagenesised with a view to isolating the corresponding genes. Joint with BB/D522089/1 BB/D522438/1