Marigold Cell Size and Polyploidy

Abstract

Most animals are diploid, having one set of chromosomes from the male and one from the female. Polyploid animals, with the exception of some frogs and fish, are usually aborted or die immediately after birth (Gardner et al., 1991). In contrast, estimates are that about 70% of flowering plants and 90% of ferns contain three or more sets of chromosomes (Masterson, 1994; Pichersky et al., 1990). Chromosomes pair at meiosis, therefore most organisms have even sets of chromosomes, such as tetraploids (4 sets), and hexaploids (6 sets). Those with odd numbers have reduced fertility (triploids for example) and often reproduce vegetatively. Many crop plants are polyploid, including coffee, cotton, potatoes, strawberries, sugar cane, tobacco, wheat and corn. Polyploidy in plants has been investigated since the 1930s to try to understand and perhaps make use of its effects (Stebbins, 1947). The grain crop triticale, for example, is a human-generated hybrid polyploid of wheat (Triticum aestivum) and rye (Secale cereale) formed by scientists containing the complete genomes of both grasses. Plant breeders induce polyploidy to attempt to increase yield, improve qualities like fruit size or vigor, and to adapt crops to particular growing conditions (Dewey, 1980; Zeven, 1980). The seedless watermelon and larger tetraploid grapes are examples. In some instances polyploidy has increased flower, seed or fruit size, increased photosynthetic or respiration rates, or increased tolerance of extreme temperatures, drought or flooding (Tal, 1980). However, there are few consistent effects, the primary one being an increase in cell size (Masterson, 1994; Bennett and Leitch, 1997). We have developed a lab (Hunter et al., 2002) based on polyploidy and cell size, to introduce middle school, high school, and college students to several important subjects in biology, including genetics (chromosomes, meiosis and mitosis, polyploidy), plant anatomy (stomata, air and water exchange, leaf structure) and cell biology (genome size and cell size). It also allows the use of simple math in data analysis and utilizes quantitative measurements rather than simple observations. The lab involves growing marigolds for about one month from seed, and measuring guard cell (surrounding the stomata) sizes and densities. A modified version of the lab was presented at the 2003 ABLE meeting in Las Vegas.