EVOLUTION 15: 394-400. (December,1961)
DIVERGENCE IN TRIPSACUM AND ZEA CHROMOSOMES1
MARJORIE P. MAGUIRE Genetics Foundation, Department of Zoology, University of Texas
Received January 4, 1961
1This work was supported by a grant from the National Science Foundation (G-7068) and by a PHS research grant (RG-6492), from the National Institutes of Health, Public Health Service.
It is generally agreed that Tripsacum and Zea have probably evolved as divergent genera from an ancient common ancestor (Mangelsdorf and Reeves, 1939; Montgomery, 1906; Randolph, 1952; Weatherwax, 1918). The mode of origin of the striking differences between their chromosomes may be elucidated by studies of hybrid backcross progenies in which Tripsacum chromosomes or segments of them are included in a genetic background which is otherwise that of Zea mays L. (Mangelsdorf and Reeves, 1939; Maguire, 1957). The substitution of a large Tripsacum chromosome segment for a portion of the corn genome has made possible a direct comparison of the effects of corresponding corn and Tripsacum segments on the development and reproduction of the corn plant. The results of this study are reported here, with a consideration of the evolutionary processes which might have yielded the differences found between these two chromosome segments.
Divergent viewpoints on the relationship of Tripsacum and corn have been reviewed recently by Mangelsdorf and Reeves (1959) and Randolph (1959). The major differences between the two genera and their crossability are of importance to this study and will be outlined briefly below.
Diploid Tripsacum has 18 pairs of chromosomes with an average length much shorter than that of the 10 pairs of corn chromosomes, and the chromosomes of Tripsacum differ from those of corn in arm ratio and other features (Longley, 1941; Randolph, 1955). Corn and Tripsacum are phenotypically very different. Tripsacum has well‑developed staminate and pistillate spikelets in all inflorescences, with the pistillate below on a distichous, disarticulating rachis. Corn normally has separate staminate and pistillate inflorescences with the staminate terminal, the pistillate lateral and enclosed by husks or modified leaves. The axis of the pistillate inflorescence of corn is polystichous and does not disarticulate. Growth habit, branching and leaf width are also very different in the two genera.
Artificial hybridization of corn and Tripsacum is difficult. When corn is used as the female parent, a few seeds (usually viable only under embryo culture) may be set following pollination with Tripsacum if the silks of corn have been shortened (Mangelsdorf and Reeves, 1939) or the pollen applied near the base of the corn silks (Randolph, 1955). Attempts to produce the reciprocal cross, using Tripsacum as the female parent, have failed except those of Farquharson (1957) who tested the crossability of Tripsacum with more primitive varieties of corn. She was able to obtain a hybrid plant which reached maturity from the pollination of Tripsacum dactyloides L. with a Peruvian stock of corn. Randolph (1952) found Mexican and Guatemalan Tripsacum and corn to be highly cross‑incompatible. All the mature hybrids of corn and Tripsacum which have been produced have irregular meiosis, do not yield functional pollen, and the only functional eggs are those which contain the unreduced complement of 28 chromosomes. Maguire (1960) found genetic recombination to be extremely rare between corresponding corn and Tripsacum segments.
The genomic differences between the two genera and the difficulty of crossing them artificially support the view of Randolph (1955) and Weatherwax (1955) that recent natural exchange of chromosome material between corn and Tripsacum has probably happened rarely, if at all, with the possible exception of South American forms (Farquharson, 1957).
HETEROZYGOUS AND HOMOZYGOUS SUBSTITUTION PLANTS
The plants studied in this experiment contained a Zea‑Tripsacum interchange chromosome in which the distal half of the short arm of corn chromosome 2 had been exchanged for a corresponding segment from a Tripsacum chromosome bearing a conspicuous terminal knob. Plants heterozygous for this chromosome (i.e., containing one interchange chromosome 2 and one normal chromosome 2) are referred to as "ZT" plants. Plants homozygous for this chromosome (two interchange chromosomes 2 present with complete exclusion of normal corn chromosome 2) are referred to as "TT" plants. Since the remainder of the complement of these plants comprises the rest of the corn chromosomes with no other apparent abnormalities or alterations, the letters "Z" and "T" in these designations refer only to the origin of the distal half of the short arm of chromosome 2. The T segment was contributed from the long arm of a chromosome derived from Tripsacum which was about half as long as corn chromosome 2 and had an arm ratio of about 3.3:1. The ZT plants were obtained from corn backcross progenies of Zea mays L.‑Tripsacum dactyloides L. hybrids as described in earlier papers (Maguire, 1957, 1960); TT plants were segregants of intercrosses of ZT plants.
The T segment in ZT plants is thought to be intact as derived from Tripsacum (or at least not modified by crossing over), since these plants were the offspring of 21-chromosome plants containing both reciprocal interchange products and one normal corn chromosome 2. In such plants the two Z segments usually synapse, leaving the T segment univalent at pachytene. This failure of Z and T segments to synapse is considered evidence that the T segment was intact (or very nearly so) in these parent plants. The T segment is probably intact in TT plants, since recombination between Z and T segments in 20-chromosome plants was extremely rare in tests involving markers located relatively near opposite ends of the region in question (Maguire, 1960). No evidence has been seen in the thousands of clear microsporocytes studied (from plants of all the various constitutions) of other types of alteration of the Z or T segments such as inversion, translocation, deletion, or duplication. Minor aberrations which might not be detectable cytologically would not be expected to cause crossover inhibition of the extent found.
The T and Z segments bear a degree of mutual homology despite the failure of crossing over between them, as indicated by the following observations. In ZT plants the interchange and normal chromosomes 2 synapse completely and with no extension of one beyond the other in about 93% of microsporocytes (Maguire, 1960). In 21‑chromosome plants containing two interchange chromosomes 2 (each with the T segment), and the reciprocal exchange chromosome (with the Z segment attached to the remainder of the original chromosome from Tripsacum), 10 bivalents and a univalent are found in about 95% of microsporocytes at pachytene. But in the rest of the cells a trivalent configuration is formed in which the Z segment of the extra chromosome synapses with one of the T segments to the exclusion of the other, which is assumed to match its partner perfectly or nearly so (fig. 1). Thus although there is preferential pairing of T with T, Z occasionally competes successfully in synapsis with T. Also, the T segment has been shown to carry normal dominant alleles for two recessives located in the distal half of the short arm of corn chromosome 2 (Maguire, 1960).
Plants of ZT constitution were obtained as described above, and these were compared with their ZZ sibs and with TT segregrants. Plants were grown side by side from seeds planted simultaneously. In all cases the constitution of the plants was determined cytologically by means of presence or absence of the terminal knob of the T segment.
|FIG. 1. A, Photomicrograph showing a trivalent configuration in a plant with two interchange chromosomes 2 (each with a T segment carrying a terminal knob) and an extra chromosome carrying a Z segment attached to Tripsacum chromosome material. This Z segment is synapsed with one T segment, while the other T segment is univalent. B, Semidiagrammatic representation of the trivalent configuration shown in A. Magnification about 1000 X.|
While ZT plants cannot be distinguished from ZZ plants on the basis of their gross appearance, TT plants have a characteristic conformation: They are short and stocky, with stiff leaves and very few tassel branches (fig. 2). Their silks are usually split (as illustrated in fig. 3) for an appreciable distance back from the tip, although this varies greatly from one plant to another (table 1). All available plants of ZT, ZZ, and TT constitution were measured and compared for a number of quantitative characteristics. The data are presented in table 2. Means and variances were calculated and t tests applied for an estimation of the statistical significance of differences of means. Because of the small numbers of plants involved in these tests and their probable genetic heterogeneity, the tests are insensitive to minor differences in effect of the Z and T segments, and are meaningful only where differences are striking.
The means of ZT and ZZ plants did not seem to differ from each other, but the means of both differed from the mean of TT plants in the following characteristics: TT plants were shorter, had fewer nodes, fewer tassel branches, a smaller number of rows of ovules, and a smaller number of ovules per row. Both ZT and TT plants showed a tendency to be proterogynous and to have narrow leaves. TT plants had more earshoots than ZZ plants, but ZT plants were intermediate in this respect, not differing from either ZZ or TT plants. ZT plants had longer leaves and more veins per centimeter of leaf width than either TT or ZZ plants. TT, ZZ, and ZT plants did not differ in number of tillers or internodal length.
It is difficult to evaluate the influence of substitution of the T segment on such characteristics as plant height (in this case apparently a direct reflection of node number), number of rows of ovules, and number of ovules per row, since the differences correlated with substitution might result from a general reduction of vigor rather than direct effect of Tripsacum genes. However, the tendencies of TT and ZT plants to be more proterogynous and of TT plants to have more earshoots and split silks are probably most easily explained as tripsacoid influence. The drastic reduction in number of tassel branches in TT plants is probably also most easily explained in these terms.
|FIG. 2. Photograph of a typical TT plant.||FIG. 3. Photograph showing silks with split tips from an ear of a TT plant.|
Pollen fertility in ZT plants was normal, or very nearly so, with about 97% large, well-filled grains of uniform size. The proportion of pollen carrying the T segment which functioned in fertilization in direct competition with pollen carrying the Z segment from these plants was about 42%. TT plants showed a wide variation in proportion of shrivelled pollen. The proportion of normal-appearing pollen found in these plants ranged from 1% to 93%, with a mean of 73% and a variance of 37. This large variability of pollen in TT plants might result from a reduced tolerance to minor fluctuations from optimal conditions in plants of unbalanced genome, as well as to differences in the genotypes of the plants. Fabergé (1944) proposed such an explanation for variable fertility in his work with Pa paver hybrids.
Perhaps the most surprising findings of these studies are that pollen with a T segment substituted for a Z segment is normal in appearance and function, and that TT plants are viable and fertile to a high degree. In other words it appears that a Tripsacum chromosome segment can be substituted for a corn chromosome segment equal to about 3% of the total length of the corn genome. Extensive deficiencies are rarely viable in corn gametophytes. To the writer's knowledge the most extensive haplo-viable deficiency was that reported by Stadler (1933) involving about 1/3 of the long arm of chromosome 10 (about 1% of the corn genome). While eggs carrying this deficiency were viable to some extent, pollen was not, and plants homozygous for it were never produced. It is probable that there are genes essential to normal development and reproduction of corn located in the distal half of the short arm of its chromosome 2, and that adequate substitutes for these exist in a Tripsacum chromosome region. Essential loci might be expected to be resistant to evolutionary change, but interchangeable arrays of such loci are not expected to exist in chromosome segments of equal length in genera whose genomes differ as widely as those of Tripsacum and corn. The question which naturally arises cannot be answered at present: Are the Z and T segments of this experiment unusual in their similarities, or are there widespread basic similarities between corn and Tripsacum chromosomes?
TABLE 1. Lengths of split portions of silks in TT plants
TABLE 2. Measurements of quantitative characteristics
(dehiscence date minus silking date)
|Plant height (m)2||TT||24||0.884||0.027|
|Number of nodes2||TT||24||6.7||0.96|
|Average internodal length (cm)||TT||24||13.3||8.3|
|Number of tassel branches2||TT||17||4.0||2.0|
|Number of tillers||TT||25||0.44||0.34|
|Number of earshoots||TT||25||2.2||0.67|
|Number of rows of ovules2||TT||20||9.4||2.58|
|Number of ovules per row2||TT||20||22.8||22.1|
|Leaf length3 (cm)||TT||23||58.94||41.7|
|Leaf width1 (cm)||TT||22||6.8||0.43|
|Vein number per cm leaf width3||TT||22||1.74||0.10|
The exchange which produced the interchange chromosomes of this experiment was probably a crossover type of event, since it involved corresponding, partially homologous segments. It seems unlikely that it was terminal with respect to regions partially homologous, and unless the exchange was terminally located, at least an additional region of undetermined length must also correspond closely in the two genera. Unfortunately only the Z and T segments of this experiment have been tested so far for this kind of similarity, and further tests must await the development of strains with other interchange chromosomes. In view of the rarity of crossing over between these two regions other exchanges probably will not be readily obtained.
If strong similarities between corn and Tripsacum chromosomes are confined to a few well-delimited segments, the theory of Mangelsdorf and Reeves (1939) will be supported to the extent that recent exchange of chromosome material between corn and Tripsacum has been of importance, however rarely it may have occurred. But if similarities of the sort demonstrated here are widespread, then chromosomes of Tripsacum must he basically more like those of corn than a comparison of their idiograms would suggest. There is evidence to support the view that 36 chromosome diploid Tripsacum actually arose as an allotetraploid (Randolph, 1955). In this case the basic genome of Tripsacum would be 9 chromosomes compared to the 10 of corn. The fact that corn chromosomes are much longer than those of Tripsacum at pachytene might be attributed in part at least to general differences in the development of the pachytene stage. Even different stocks of corn differ widely in the overall length of their chromosomes at pachytene, although the various members retain the same relative lengths with respect to each other. Is it conceivable that the differences between the basic corn and Tripsacum genomes may be traced to a few major and many minor chromosome aberrations? If so we must assume that the meiotic irregularities of the corn-Tripsacum hybrid are due to incompatibility factors, general unbalance, or possibly to synaptic or crossover barriers, rather than to wide differences in chromosome homology. Unfortunately, it has been impossible to date to prepare an analyzable pachytene slide from a corn-Tripsacum hybrid for direct study of synaptic relationships. Since crossover barriers exist between regions partially homologous, the failure to find bivalents at later meiotic prophase of the hybrid is inconclusive.
In any case chromosomal rearrangements must have been minor within the Z and T segments studied in this experiment. The differential changes responsible for their differing phenotypic effects, for the failure of crossing over between them and synaptic failure between them under some conditions must have been limited to change in nonessential loci. These non-essential loci available for differential change might have been already present in the common ancestral chromosome segment, or produced by duplication or rearrangement of essential loci in the course of divergence.
A segment from a Tripsacum chromosome has been substituted for the distal half of the short arm of corn chromosome 2 in plants whose chromosomes are otherwise apparently unaltered corn chromosomes. Plants both heterozygous and homozygous for this substitution have been obtained. Heterozygous plants are indistinguishable from normal corn in gross appearance, although they seem to differ somewhat in certain quantitative measurements. Plants homozygous for the substitution have a characteristic appearance, and differ more widely from normal corn in quantitative measurements. Both heterozygous and homozygous plants are vigorous and fertile. Since cytological and genetic evidence seem to support the view that the Tripsacum segment has remained intact, it is concluded that this segment probably contains all of the loci essential to the normal development and reproduction of the corn plant which are located in the distal half of the short arm of its chromosome 2. Processes are discussed which might have contributed to the similarities and differences of these corresponding segments in the two divergent genera.
The writer is grateful to Dr. L. F. Randolph for constructive criticism of the manuscript and helpful suggestions.