Tight clustering and hemizygosity of apomixis-linked molecular markers in Pennisetum squamulatum implies genetic control of apospory by a divergent locus that may have no allelic form in sexual genotypes

Abstract

Apomixis is a naturally occurring mode of reproduction that results in embryo formation without the involvement of meiosis or fertilization of the egg. Seed-derived progeny of an apomictic plant are genetically identical to the maternal parent. We are studying a form of apomixis called apospory that occurs in the genus Pennisetum, a taxon in the grass family. A cultivated member of this genus, pearl millet (Pennisetum glaucum), reproduces sexually. A wild relative of pearl millet, Pennisetum squamulatum, that is an obligate aposporous species, is cross-compatible with pearl millet when used as a pollen donor in the interspecific cross. We present herein the genetic mapping of 13 molecular markers in an interspecific hybrid population of 397 individuals that segregates for apomixis and sexuality. Surprisingly, 12 of the 13 markers strictly cosegregated with aposporous embryo sac development, clearly defining a contiguous apospory-specific genomic region in which no genetic recombination was detected. Lack of or suppression of recombination may be coincidentally associated with the chromosomal context of the apomixis locus or it may be a consequence of its evolution that is essential for preservation of gene function as has been previously shown in studies of complex loci in both plant and animal species.

Figure 1

(A) RAPD pattern (lanes 1–8) with primer P16 and pooled DNA from apomictic and sexual individuals and the 950-bp SCAR (lanes 9–16). Each lane represents four genotypes bulked by phenotype (apomictic or sexual). (B) A blot of the gel shown in A probed with the cloned P16 RAPD fragment.

Figure 2

A genomic DNA digest (DraI) of P. squamulatum (first lane of apomicts), P. glaucum (first lane of sexuals), and eight F₁ individuals of each reproductive phenotype probed with ugt197 (A), C4–600 (B), Q8M-800 (C), A14M-200 (D), and O7M-600 (E). The same blot was stripped and reprobed with each sequence. All probes detected either hemizygous (A–D) or polymorphic (E) DNA sequences in the apomictic individuals. None of the probes hybridized with bands of the same molecular weight.