Early events in the evolution of the Silene latifolia Y chromosome: male specialization and recombination arrest

Abstract

Understanding the origin and evolution of sex chromosomes requires studying recently evolved X-Y chromosome systems such as those in some flowering plants. We describe Y chromosome deletion mutants of Silene latifolia, a dioecious plant with heteromorphic sex chromosomes. The combination of results from new and previously described deletions with histological descriptions of their stamen development defects indicates the presence of two distinct Y regions containing loci with indispensable roles in male reproduction. We determined their positions relative to the two main sex determination functions (female suppressing and the other male promoting). A region proximal to the centromere on the Y p arm containing the putative stamen promoting sex determination locus includes additional early stamen developmental factors. A medial region of the Y q arm carries late pollen fertility factors. Cytological analysis of meiotic X-Y pairing in one of the male-sterile mutants indicates that the Y carries sequences or functions specifically affecting sex chromosome pairing.

Figure 1.—

The S. latifolia sex chromosomes and the current sex determination model according to Negrutiu et al. (2001) and Charlesworth (2002). Y-chromosome deletion mutants (Westergaard 1958; Donnison et al. 1996; Farbos et al. 1999; Lardon et al. 1999; Lebel-Hardenack et al. 2002; Zluvova et al. 2005a) define at least two critical sex-determining regions on the differential arm of the Y-chromosome: the regions responsible for suppressing female organ development and for early stamen development (see text), while the arm homologous with the X (i.e., containing the PAR) carries a region responsible for late stamen development (plants lacking this region are male sterile). When compared to the known gene order on the X chromosome, the presence of an inversion can be deduced. Molecular markers are listed on the right, those present both on the X and Y chromosomes are in bold. The p and q arms are indicated.

Figure 2.—

Meiotic preparations of wild type and two Y-deletion mutants showing altered X–Y behavior in meiosis. Wild-type diakinesis. Note the terminal pairing of X and Y (A). Diakinesis in ad2 mutant in which X and Y do not pair. Note that the distal regions of the X are not aligned with any of the Y distal regions (B). Diakinesis in ad7 showing pairing of X and Y. One distal region of each chromosome is aligned (C). White arrowheads indicate X–Y chromosome pairs.

Figure 3.—

Outline of the deletions observed in the mutants analyzed. The markers are shown at the top, ordered according to the Y map (Hobza et al. 2006), with inverted triangles indicating the sex determination functions located on the p arm and a pollen fertility region on the q arm. The pseudo-autosomal region is on the right. The mutant names are given on the left. The underlined mutant names state for mutants with pairing phenotypes. Full lines indicate markers present in each deletion mutant, dashes indicate markers that were determined to be missing, and dotted lines indicate that the presence of the marker was not tested in the strain. Deletions are not to scale. Note that in several mutants more than one marker is deleted.

Figure 4.—

Histological analyses of S. latifolia wild type and Y chromosome deletion mutants. An outline of anther development is given on the left. The middle column shows sections of wild-type anthers, and the right-hand column displays sections of anther developmental mutants. Detailed descriptions of the wild-type anther development and Y-chromosome deletion mutant histology are given in supplemental Figures S1 and S2 at http://www.genetics.org/supplemental/. The anther rudiment in female plants stops developing before the formation of the parietal layers (A). The anthers of the Y chromosome deletion mutant asexua 1 (asx1) strongly resemble female anther rudiments (B). The inner subwhorl anthers of the anther development mutant ad1 also do not develop beyond this stage, probably because of the accumulation of deposits around the connective (C). Wild-type male anthers develop the primary parietal layer and the sporogenous tissue (D). In the anther development mutant ad6, the primary parietal layer is altered, but the sporogenous tissue is able to form microspore mother cells (E). In the ad3 mutant, abaxial locule development is arrested prematurely (F). Unlike the wild-type anthers, where no difference in the staining ability of the secondary parietal layers is observed (G), the inner secondary parietal layer of the ad7 mutant differentiates into a densely-stained tapetum-like layer (H). In the wild-type anthers, the tapetum becomes densely stained after the formation of all anther layers, and this process is connected with its differentiation (I). However, the tapetum is not stained in the pollen fertility mutant pf1 (J). During meiosis and tetrad formation, the tapetum starts to degenerate and its cells are released from each other but remain attached to the other anther layers (K). In the pf2 mutant, the tapetum is released from the anther layers and forms an agglutinate inside the anther (L). The wild-type microspores, when released from the tetrads, are round shaped (M), but in mutant pf3, the microspores are irregular in shape (N). Mutant pf4 forms round microspores, and the endothecial and tapetal cells do not degenerate, as deduced from the fact that the endothecium is present and the tapetal cells remain attached to each other (O). When compared to the wild type (P), both the anther locules and the pollen grains of the outer subwhorl of the mutant ad1 are abnormally small (Q). dep, Deposits; epid, epidermis; 1° par, primary parietal layer; spor, sporogenous cells; mmc, microspore mother cells; ab, abaxial locule; ad, adaxial locule; 2° par, secondary parietal layer; tap, tapetum; mi, microspores; pg; pollen grains. Bars, 10 μm (A–C, E, H, J, O), 25 μm (D, G, I, K–N, P, Q), and 50 μm (F).