4. The origin of the perennial rhizome habit of Euchlaena perennis Hitch. has puzzled students of species relationship in the tribe Maydeae for many years. All other American representatives of the tribe are annuals, with the exception of Tripsacum, which is perennial but grows in dense clumps and has very short rhizomes unlike the elongate freely-spreading rhizomes of perennial teosinte. The annual teosinte of Central America and Florida that has been examined cytologically is diploid. The perennial teosinte, known only from one very restricted area in Mexico, is tetraploid and has multivalent synapsis of its chromosomes and other characteristics which indicate that it is either a true autotetraploid or an allotetraploid of two closely related species or ecotypes.
Diploid forms of perennial teosinte and tetraploid forms of annual teosinte are unknown in nature. However, a somatic mutation from the annual to the perennial habit occurred in a plant of Durango teosinte grown in the greenhouse in 1931. The annual portion of this plant (1359-10) was diploid and its selfed progeny were diploid annuals with the exception of one plant (1625-B-1). which was tetraploid and perennial. The perennial rhizome sector of plant 1359-10 was propagated vegetatively, and several root-tips collected from it soon after it was discovered were examined cytologically and found to be entirely tetraploid. However, of 15 seedlings produced during the following flowering period from selfed seed of the perennial mutant one was triploid and 14 were tetraploid, and the mutant pollinated during this same period by tetraploid corn produced 11 tetraploids and one triploid, suggesting that diploid tissue persisted in the mutant sector up to the time the first crop of seed was produced sufficient to form at least 2 female gametes with a monoploid set of chromosomes.
The spontaneous occurrence of this somatic mutation from the annual diploid to the perennial tetraploid condition was interpreted as strong evidence in support of the assumption that E. perennis was simply a tetraploid mutant of E. mexicana.
To test this assumption further, tetraploidy was induced experimentally in stocks of Durango, Chalco and Florida teosinte with the heat-treatment technic. These artificial tetraploids had the annual growth habit of the parent diploids and exhibited no perennial characteristics whatever.
Another test of the relation between tetraploidy and the perennial habit involved the identification of parthenogenetic diploids in the progeny of E. perennis to determine whether they would be annual or perennial. In diploid maize parthenogenetic haploids occur with an average frequency of about 1:2000, and in tetraploid maize parthenogenetic diploids occur with an average frequency of about 1:1000. Data from greenhouse material of perennial teosinte (teosinte is a short-day plant which normally flowers during November in this latitude) accumulated during the past 10 years indicate that haploid parthenogenesis is extremely rare in this species, In this experiment, the results of which are summarized in the accompanying table, various stocks of perennial teosinte were used, including a culture from rhizomes collected at the type locality in Mexico (E16-515), a seedling from seed harvested from the type material in Mexico (E13-533), selfed progeny of E16-515 (2660), selfed progeny of E13-533 (2661), the spontaneous tetraploid mutant (1359-10) and the tetraploid seedling (1625 B-1) from the annual portion of this plant.
Seedling progenies obtained from various perennial
teosinte × diploid corn crosses, 1932-1941
Perennial teosinte stocks
Perennial teosinte is propagated vegetatively with the greatest of ease and no difficulty is experienced in maintaining individual clones indefinitely. To facilitate the identification of parthonogenatic individuals in the seedling stage, the perennial teosinte stocks were crossed with corn pollen of the constitution A B Pl C Rg Pr or a B Pl lg. The triploid hybrid seedlings of these crosses would be purple and under suitable cultural conditions could be distinguished readily from maternal, weak sun-red seedlings. Parthonogenetic seedlings of paternal origin would be either purple with green anthers at maturity or green liguleless. One parthenogenetic maternal diploid and one parthenogenetic paternal haploid were identified among the 29,861 seedlings from the perennial teosinte × diploid corn crosses grown during the period from 1932 to 1941 inclusive. The maternal diploid appeared in the 1936 progeny of culture 2661, which in that year contained 1156 seedlings. This exceptional diploid had the annual growth habit. It tillered profusely, but produced no rhizomes and after forming a few aborted tassels the plant died at about the same time annual teosinte plants of the same age mature and then die. The parthenogenetic haploid of paternal origin had narrow leaves and otherwise resembled teosinte in the early seedling state, except that it was a diminutive seedling and had the purple color of the pollen parent; later in ontogeny it became typically maize-like and was indistinguishable from ordinary maternal haploids of the same stock.
In addition to these trio exceptional seedlings there occurred each year a small number of maternal tetraploid seedlings. These were at first assumed to be contaminations, but the prevalence among them of recessive chlorophyll mutants suggested that at least some of them may have originated from unfertilized, normally-reduced diploid eggs followed by chromosome doubling in early embryogeny. If this is happening, it would help to explain the low frequency of maternal diploids obtained from this perennial teosinte × corn cross.
The perennial rhizome habit of E. perennis does not behave as a simple Mendelian recessive. The F1 perennis × 4n corn is intermediate in that it can be maintained by careful subdivision and occasionally produces short rhizomes. The character does not segregate sharply in F2 and back-cross progenies but behaves like typical quantitative characters that are dependent on the interaction of multiple factors. In these segregating progenies most of the plants tillered much more profusely than did the 4n corn parent, but very few developed any appreciable rhizome system during the summer season. A much longer growing season than we have at Ithaca is needed to make really satisfactory classification for rhizome habit in material of this kind. However, it is apparent from the general character of the segregating populations and the intermediate nature of the F1 plants with respect to rhizome habits that a dosage effect is involved, and it is therefore conceivable that cumulative gene action accompanying chromosome doubling might transform an annual into a perennial in the presence of a suitable genotype.
Some such interpretation of the origin of the perennial rhizome habit of E. perennis is supported by the occurrence of the parthenogenetic maternal diploid lacking the perennial rhizome habit in the progeny of E. perennis, and by the occurrence of the spontaneous perennial, tetraploid chimera in an annual plant of E. mexicana. The persistence of the annual habit in the experimental autotetraploids of E. mexicana may mean that the stocks from which they were produced lacked the essential genes requisite to the production of the perennial habit in the tetraploid state. It is generally believed that most annual forms of teosinte possess admixtures of maize genes. This would provide ample opportunity for displacement of genes of annual teosinte having perennial prepotencies by maize genes with strong annual prepotencies and would account for the appearance of the perennial habit in some annual teosinte tetraploids and not in others.
L. F. Randolph