THE DEVELOPMENT OF STERILE OVULES IN LYCOPERSICON ESCULENTUM MILL

PLANTS THAT are unfruitful for cytogenetic reasons appear in field populations of horticultural tomatoes (Lycopersicon esculentum Mill.) at the rate of about one per 1,000 fruitful plants. Abortion of pollen or ovules largely explains the barren condition of these plants. The gametic sterility, in turn, may reflect a breakdown in development of the gametophyte-a breakdown caused by recessive genes in diploids or by unbalanced chromosomal complements in heteroploids. The causes of sterility in a random sample of unfruitful plants were recently investigated (Rick, 1945). In the cytogenetic survey made, functionless ovules were found to consist of two strikingly different types, whose development is described in the present report. Thin sections of ovaries at anthesis or at earlier developmental stages were fixed in form-aceticalcohol (formula used by Cooper, 1931). After imbedding in paraffin and sectioning, the material was stained in iron-alum haematoxylin according to Esau's (1944) schedule. Ovaries fixed at anthesis provide the data in table 1. Except where noted otherwise, ovaries were collected in the late summer from plants growing in the field at Davis. A very high proportion of normal ovules was characteristic of ovaries collected from fruitful plants at this time. That conditions were also favorable for fruiting is indicated by the fact that unfruitfulness was observed only in plants that were cytogenetically sterile. Appearance of fertile and sterile ovules at anthesis.-A fertile tomato ovule contains an embryo sac that is highly vacuolate and greatly distended. The embryo sac is broadly ellipsoidal, its dimensions in these preparations averaging 41 X63 [s. The same features are found in freehand sections of living ovules. Since the nuclei of the embryo sac are distributed in various planes throughout this large cavity, it is seldom possible to find all nuclei in a single microtome section. Despite this enlarged condition, one can readily identify normal embryo sacs by scanning serial sections. The synergid cells and polar nuclei are shown in one plane in figure 6. The synergid cells are characteristically cuneate in shape. At anthesis the nucleus appears at the narrow micropylar end of the synergid cell, while a single vacuole occupies the other broadened end. The egg cell is usually in contact with the synergids, often surmounting their vacuolate ends. The polar nuclei, which at anthesis may exist in separate (fig. 6) or fused condition, usually appear midway between the micropylar and chalazal ends of the embryo sac and are found mostly in the peripheral cytoplasmic layer. Occasionally, however, they 1 Received for publication December 10, 1945. may be observed in the center of the large vacuole, connected with the periphery by thin cytoplasmic strands. The three antipodal cells disintegrate during the later stages of maturation; as a rule, only vestiges of them remain at anthesis. The nucellar epidermis, which collapses earlier in development, is recognized at anthesis only by a thin layer of crushed cell walls immediately surrounding the embryo sac. The remains of the nucellar epidermis are surrounded by densely cytoplasmic, parenchymatous, integumentary cells, roughly isodiametric in shape. The main body of the ovule consists of similar, though more vacuolate, cells. The single integument is penetrated by a micropyle, whose length averages 125 su. The sterile types contrast sharply in appearance with normal ovules. In one, arbitrarily called the collapsed type (fig. 11, 12, 20), the embryo sac cavity is variously expanded at anthesis, but is never so large as in normal ovules. Ovules of the collapsed type can be identified easily by this difference in size of the embryo sac cavity. Instead of the structures described above, small, dark-staining, irregularly shaped masses occupy the cavity. These contents-judging from their degenerate and amorphic condition are probably the remains of gametophytes that collapsed before maturity. Studies of development bear out this assumption. A nucellar epidermis in various stages of collapse can always be distinguished. Occasionally an ovule at anthesis contains a gametophyte in some immature stage of development. Figure 19 illustrates a gametophyte that persisted to anthesis in the single-megaspore stage. Development is often arrested in the subsequent two-nucleate stage. Although these immature structures appear to be healthy, it is doubtful whether they would ever develop into mature embryo sacs before fertilization. Being examples of abnormal gametophyte development, the immature types are grouped with the collapsed type. The other type of sterile ovule presents a rather enigmatic appearance and differs markedly from the collapsed and normal types. The cavity normally occupied by a fully developed embryo sac is filled, in this instance, by a mass of 20 to 40 relatively undifferentiated cells (fig. 17, 18). Being more vacuolate, these cells are conspicuous in contrast to the more densely cytoplasmic cells of the surrounding integument. This structure may vary in shape from narrow and elongate (fig. 17) to ovoid (fig. 18). Its volume is always considerably less than that of a normal embryo sac. Neither the vestiges of a nucellar epidermis nor any other structure separates this mass of cells from the inner layer of the integument. This very fact suggests