University of Iowa
Ostergren (Bot. Notiser, 1945) proposed that B chromosomes are parasitic entities. If that hypothesis is correct, a relatively simple scheme for evolution of the maize B chromosome can be proposed using existing evidence. Steps in the sequence are given below:
a. A fragment chromosome developed from one of the maize chromosomes. The mechanism for its origin is unknown, but Peeters and Wilkes (MNL 57:19, 1983) give one hypothesis.
b. A specific gene on the fragment produced a beneficial effect in extra dosage, allowing selection of the chromosome. Brewbaker and Natarajan (Genetics, 1960) constructed a fragment chromosome in Petunia with a gene that confers a dosage-dependent selective advantage.
c. Most genes on the nascent B chromosome gave a negative dosage effect, as expected. Harmful effects of partial trisomy occurred in the sporophyte. Also, transmission of the fragment chromosome was limited to the female, due to genetic imbalance in the pollen. As a result, most genes on the B were inactivated through random mutation or heterochromatinization plus selection.
d. Due to its low frequency, the early B chromosome was usually found in plants as a single extra chromosome. During meiosis, univalent B chromosomes had difficulty disjoining in anaphase I and were frequently lost. Meiotic loss caused selective pressure for evolution of a system which allows regular migration of univalent B's to one pole. Two regions have been identified on the B which suppress meiotic loss, apparently by enhancing polar movement of univalents (Carlson, Crit. Rev. Pl. Sci. 1986; Carlson, MNL, 1988).
e. The nondisjunctional mechanism of the B resulted from activation of the system which suppresses meiotic loss at the second pollen mitosis. The migration of a chromosome (with two chromatids) to one pole in meiosis is identical to the process of mitotic nondisjunction. Consequently, a single genetic system was able to suppress losses at meiosis and produce nondisjunction at the second pollen mitosis.
f. Nondisjunction at the second pollen mitosis initially served as another means for circumventing meiotic loss. In crosses of 0B x 1B plants, many of the progeny receive two B's due to nondisjunction. The frequency of univalent B chromosomes in a population was thereby reduced.
g. The completion of an accumulation mechanism, through addition of preferential fertilization, occurred later. It was selected in order to maximize the frequency of B chromosomes in populations.
h. Over time, the beneficial function of B chromosome trisomy may have become unimportant. Perhaps it was superseded by duplication of the relevant gene within the standard genome. In any case, the beneficial function was lost by random mutation. The B chromosome was established as a parasitic entity.
Several variations on this proposal are feasible. For example, the modern
B chromosome may retain a beneficial function and not be truly parasitic.
Also, preferential fertilization may not have required a separate evolutionary
step. Preferential fertilization may be an inherent property of maize pollen
which occurs when the two sperm have a differential chromosome content.
Despite these uncertainties, the proposal does provide a logical evolutionary
sequence for origin of the maize B chromosome. Some of the ideas discussed
should be applicable to evolution of B chromosomes in other organisms.
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