Edmund W. Sinnott

A Developmental Analysis of Inherited Shape Differences in Cucurbit Fruits

Fruit shape in the Cucurbitaceae is inherited, and there are at least four quite distinct types of shape determination, as follows: (1) Differences in shape index (ratio of length to width) which are established in the earliest visible primordia and may persist unchanged throughout development. (2) Constant differences in growth rate between length and width which occur during the entire course of development, causing a progressive change in index. The relative growth of these two dimensions may be measured by a constant. In the types studied this varies from .8 (length growing .8 times as fast as width) to 2.2 (length growing 2.2 times as fast as width). (3) Differences in shape index at maturity which necessarily result from differences in size in all cases where the relative-growth constant is other than 1. Where shape index is progressively changing, the point at which growth of fruit stops (its mature size) will affect its index. (4) Differences in pattern or profile, which are more complex than those involving index alone. Specific patterns are inherited and are determined independently of differences in index. All four of these aspects of shape are independent of each other in inheritance. The genes which control them evidently differ in the time at which the major effect is produced and in the character of the effect itself. A purely descriptive developmental analysis is a necessary preliminary to any complete genetic or morphogenetic study of characters of size or shape.

The Relation between Body Size and Organ Size in Plants

1. The problem of the relationship between the size of the plant body and the size of the organs it produces has been studied by various workers, who find that in most cases there is a small but significant correlation between these characters. 2. In a series of bean plants, the coefficients of correlation were determined between plant size, as measured by dry weight of shoot, dry weight of fruit, number of leaves, number of pods and number of seeds; and the average dry weight per plant of leaf, pod and seed. A positive and significant correlation, though usually a small one, was found in each case. 3. An examination of the curve of means for organ size on plant size shows that in each case the curve rises steeply at first and then flattens out. In other words, an increase in the size of the plant is accompanied by an increase in the size of its organs if we consider comparatively small plants only; but after a certain size is reached, any further increase in plant size is not followed by increase in organ size. Separate correlations between plant size and organ size made for small plants (those below the point where the curve flattens) and for large ones (those above it) showed a very decided correlation in the former and practically none at all in the latter. 4. These facts suggested that the size of an organ may not be correlated with body size at all, but rather with the size of the axial growing point from which it develops. In support of this idea attention is called to the fact that during the early stages of a plants growth there is up to a certain point a progressive and parallel increase in the size of the plant and in the size of the primary meristems of its axes; but that after this point is reached meristem size remains constant and further increase in body size is not accompanied by any increase at all in that of the growing point. 5. Favorable material to test this hypothesis directly was afforded by twigs and leaves of Acer saccharum. The correlation between the blade volume of a given leaf pair and the cross-sectional area of the pith of the internode below (used as an index to the size of the growing point from which the leaves had developed) was found to be high (+ .807 ± .024). 6. It is therefore concluded that the size of a plant organ (leaf, fruit or seed) is dependent not upon the body size of the plant on which it is borne, but rather upon the size of the growing point from which it developed.

Factors Determining Character and Distribution of Food Reserve in Woody Plants

1. Previous observations upon the character, distribution, and seasonal changes of the food reserves of woody plants in temperate regions were in general confirmed by the present investigation and were considerably extended. 2. A study of the minute distribution of the food reserves in the tissues of the stem (twigs and young branches) during the winter shows that (1) starch is commonest in regions remote from centers of conduction and in cells with thick, well lignified, small-pitted walls; and (2) fat is most abundant in and near the phloem, close to vessels, and in cells with thin or unlignified walls or large pits. 3. These facts indicate that the character of the food reserve in any cell depends primarily upon the ease with which water or substances carried by water have access to the cell. Where the movement of liquids is apparently slow and difficult, the reserve persists as starch; where such movement is easy, starch disappears at the beginning of winter and fat is produced. 4. This suggests that differences in the type of food reserve may be due to (1) differences in water content of the various storage cells, resulting in modification of enzyme activity, or (2) differences in the ease with which enzymes have effective access to the storage cells.

Developmental History of the Fruit in Lines of Cucurbita pepo Differing in Fruit Shape

1. The inheritance of fruit shape in Cucurbita pepo may be ascribed to the operation of Mendelian factors. 2. In inbred and genetically pure lines showing the three main shapes of fruit (elongate, spherical, and disk), comparative studies were made of the developmental history of the various shape types from the earliest floral primordium to maturity. 3. From a very early stage in differentiation, when the ovary primordium is not more than one-millionth of the volume of the mature fruit, each of the various shape types is clearly distinguishable. 4. Much of the difference between the types, especially between the elongate and the spherical, was found to be due to shape differences involving only the central carpellary tissue (later the seed cavity) and not the wall of the ovary and fruit, the latter maintaining an essentially constant width in fruits of diverse shapes. 5. In most of the disk types, however, not only is the seed cavity relatively short and broad, but there is also a localized thickening of the wall in the equatorial zone (involving only the inner wall layers) which results in a still flatter shape for fruits of this type. 6. In certain lines a sterile and relatively slender neck is developed at the base of the ovary. 7. The various fruit shapes are essentially similar in the size and shape of their component cells. 8. The factors determining shape are evidently those which govern growth correlation. They operate by controlling: (1) cell polarity, and thus the plane of cell division; and (2) the localization of growth in particular regions.

THE EFFECT OF GENES ON THE DEVELOPMENT OF SIZE AND FORM

1. Genetics has concerned itself almost entirely with an analysis of the hereditary constitution of organisms in terms of genes. For a complete understanding of hereditary differences, however, it is important to determine how genes control the processes of development so that specific characters are produced. Traits of size and form lend themselves most readily to studies of this sort, and in a few instances direct evidence of changes in development brought about by specific genes has been obtained from both animals and plants. This evidence is briefly reviewed.

Factorial Balance in the Determination of Fruit Shape in Cucurbita

1. Several independent genetic factors tending to flatten fruit shape in Cucurbita have been isolated. Their recessive allelomorphs produce various elongate shapes. 2. Other elongate types are evidently not due to factors of this sort but to others which either produce shape elongation directly or inhibit the operation of flattening factors. 3. Genetic analyses of certain pure lines of Cucurbita with elongate fruits, based on a study of crosses between these and flatter lines of known constitution, are described. These indicate that in certain lines there is one elongating factor of this sort, whereas others evidently possess two or more. 4.A specific fruit shape seems to be the resultant of a balance between a series of factors differing in the degree and direction of their effect. Shapes which are phenotypically identical may be very diverse genotypically. 5. These shape factors evidently operate through their influence on growth correlations.

Comparative Rapidity of Evolution in Various Plant Types

1. The most recently evolved element in the floras of temperate North America and of Europe, as determined by a study of the indigenous endemic genera, is composed almost entirely of plants which are herbaceous in habit. 2. Herbs tend to be grouped in fewer and larger genera and families than woody plants. 3. It is therefore concluded that herbaceous plants, presumably because of the brevity of their life cycle and the rapid multiplication of generations consequent thereto, are in most cases undergoing evolutionary development much more rapidly than are trees and shrubs. 4. From this conclusion are drawn inferences as to the origin of the herbaceous habit and the antiquity of the Angiosperms.

Substance or system; the riddle of morphogenesis.

IT has often been said, and truly, that in the problems of morphogenesis, of the controlled formativeness of living matter, center the most important concerns of biological science. A tiny fertilized egg, dividing first in this plane and then that and building up a group of cells which in their growth march steadily on to a determinate organic whole, presents a problem which must ultimately involve -not only embryology but most of the other disciplines which deal with protoplasm and its activities. The mechanisms which control this orderly process of development, and which bring it back to normal paths again when accident or experimental change disturb its regular course, are little understood, though since the days of Aristotle the best minds of our science have pondered the problems which are here presented. To many biologists, aware of the chemical basis of all life and of the profound effects which changes in the chemical environment may produce on living things, it has seemed clear that specific substances must regulate development and control its course. Sachs long ago presented evidence that definite organ-forming substances determine the structures which are produced by plants. This conclusion was strengthened by other facts, such as the important influence on plant growth exerted by particular chemical elements in the. soil. Similar observations were reported in animal development, as in the 1 Address of the retiring president of the American Society, of Naturalists presented at its annual meeting in St. Louis in March, 1946.

The Genetic Control of Developmental Relationships

IN the very difficult problem of determining the mechanism by which genes control development and determine those traits in the adult by which genie differences are recognized, the first step toward a solution must evidently be to describe exactly the course which such traits follow in development. Just what a gene controls must be learned before we can understand how the gene controls it. Unfortunately in many of the traits best known genetically, notably differences in color, such a procedure is almost impossible at present because of the difficulty of tracing more than the last feew steps in the chain of reactions between gene and character. Traits of form, however, offer peculiar advantages in such a problem, since many of them may be observed and measured throughout practically the entire course of their development. Especially favorable material is provided by the multiple organs of plants, such as leaves and fruits, for large numbers, genetically identical, may be obtained from the same individual. In the family Cucurbitaceae (the squashes, gourds, melons and similar plants) there is a very great diversity in fruit shape, and in several of its members, notably Lagenaria vulgaris and Cucurblita Pepo, this occurs even within a single species. In a wide range of types, measurements have been made of the length (polar diameter) and width (equatorial diameter) of ovary and fruit from very early primordia, microscopic in size, to maturity.

Shape Changes During Fruit Development in Cucurbita and their Importance in the Study of Shape Inheritance

(1) During fruit development from early ovary primordium to maturity, in disk fruited and sphere fruited plants of Cucurbita pepo, there is a slight progressive increase in degree of flattening, or ratio of equatorial diameter to polar diameter. (2) Fruits will stop growth and become mature at widely different sizes if environmental conditions vary or if there is segregation for size factors. The mature fruit retains the shape index characteristic of the developmental stage which it had attained when growth ceased. Thus the segregation of genetic factors controlling shape is rendered much less distinct if there are size differences in the population. (3) By determining shape indices for small ovary primordia instead of for mature fruit, a population segregating for shape may be studied at a uniform size and the variability due to size differences may be eliminated. This method has the added advantages of making possible a much larger number of determinations for each plant, and of eliminating minor shape differences which often appear during development. (4) In three F2 pedigrees showing monofactorial segregation for disk and sphere fruit shape, a comparison of the plotted indices of young primordia, 1 to 8 cc in volume, with those of mature fruits from the same plant showed in every case a much sharper segregation for shape in the primordia than in the fruits. (5) It is believed that this method of studying earlier stages in development will make possible a more accurate analysis of shape inheritance in cases where the genetic situation is more complex.