Synaptic responses to concerted genomic evolution in Lathyrus. I. Intergenomic effects

Abstract

C0 autotetraploids have been derived from seven diploid (x = 7) species of Lathyrus, varying in genome size (11-20 pg DNA/2C) and content of moderately repetitive DNA (5-12 pg/2C). Pairing patterns at metaphase-I in 942 pollen mother cells (PMCs), each with a minimum of 28 chiasmata, were compared with a model relating pairing autonomy to genomic organization. Autotetraploids of one species, L. aphaca, form on average one multivalent more than those of the other six (5.2 vs. 3.7-4.2/PMC). With the exception of L. aphaca, intergenomic differences in bivalent frequency and incidence of synaptic exchange correlate with the size of the genome, particularly that of the moderately repetitive component. Such nucleotypic correlations appear to reflect losses in numbers of autonomous synaptic sites, concomitant with concerted reductions in the size of the genome. They also indicate that autonomous synapsis may be governed by moderately repetitive sequences.

Synaptic responses to concerted genomic evolution in Lathyrus. II. Intragenomic effects

“…Thus, in Lathyrus, while differences in the size of the genome reflect differing amounts of moderately repetitive DNA, differences in the size of the chromosome mask similarities in the content of moderately repetitive sequences. Our earlier results indicated that a reduction in the size of the genome is accompanied by a loss of autonomous synaptic sites (Wallace & Callow, 1993). The observations reported here suggest that intragenomic differences in the size of the chromosome have not resulted in different numbers of antonomous synaptic sites because each tetrasome within an autotetraploid nucleus has a similar propensity for quadrivalent formation and for synaptic exchange.…”

“…As in our previous paper (Wallace & Callow, 1993), pairs of univalents were counted as pachytene bivalents, while univalents with trivalents were counted as pachytene quadrivalents.…”

“…In the previous paper in this series, we reported on studies of pairing behaviour in C0 autotetraploid Lathyrus species and concluded that intergenomic differences in numbers of moderately repetitive sequences are paralleled by different numbers of autonomous synaptic sites (Wallace & Callow, 1993). Here we address the question of how pairing autonomy responds to chromosomal variation within each genome.…”

“…While the first could be observed directly, the minimum number of synaptic exchanges had to be inferred from metaphase multivalent structure (see Wallace & Callow, 1993 Where each of the seven tetrasomes within a tetraploid nucleus of Lathyrus has the same propensity to form pairs of bivalents at pachytene and the same propensity to form quadrivalents, the pairing patterns within cells will conform to a binomial frequency distribution. Thus, given that seven tetrasomes either form pairs of bivalents with probability p or quadrivalents with probability q, then pairing patterns within cells will conform to the expansion of (p + q)7.…”

“…The cells were selected from a sample of 2299 PMCs for the presence of at least 28 chiasmata in order to minimize lapses of pachytene pairing due to insufficient chiasma formation (see Wallace & Callow, 1993). Analyses of intergenomic variation are supplied elsewhere, as are details of cytological and horticultural procedures (Wallace & Callow, 1993). Estimates of genomic size are those of Narayan(1982).…”