HAUBER, DONALD P.* AND STEPHEN M. STACK. Department of Biological Sciences, Loyola University, New Orleans, LA 70118 and Department of Biological Sciences, Colorado State University, Ft. Collins, CO 80523. - Synapsis in an autotetraploid.
The autotetraploid model proposed by R.C. Jackson and colleagues
distinguishes autotetraploids from segmental allotetraploids in that
the frequencies of late prophase I circle and chain quadrivalents and
bivalents are predictable in autoploids while they are not predictable
in segmental alloploids because the degree of genome divergence is
generally unknown. Included among the tenets upon which the
autotetraploid model is based are that 1) there is one synaptic
initiation site per chromosome arm; 2) any chromosome arm has an equal
probability of synapsing with any other homologous arm; and 3)
synapsis in one arm is independent of synapsis in the opposite arm.
This model predicts that for four homologous chromosomes, a
quadrivalent will be formed 2/3 of the time and two bivalents will be
formed 1/3 of the time during the pairing stage of mid-prophase I when
the synaptonemal complex (SC) is formed. Natural tetraploid
populations of Machaeranthera pinnatifida ( =Happlopappus
spinulosus) (Asteraceae) (2n=4x=16) occurring along the Arkansas
River in south-central Colorado were previously established to be
autotetraploid based on the application of the autotetraploid model to
late prophase I meiotic configuration frequencies. The objective of
the current study was to determine if the tenets above hold true by
analyzing synapsis in M. pinnatifida autotetraploids using the
synaptonemal complex spreading technique. The quadrivalent frequency
from SCs was 0.69, which is not significantly different from the 0.67
expected. There were no significant differences in quadrivalent
frequencies among the four chromosomes. Contrary to the expectation
based on the autotetraploid model of one partner switch per
quadrivalent, we observed a range of 1-4 partner switches with an
overall mean of 1.56. This suggests that the number of synaptic
initiations is greater than two per chromosome and questions one of
the primary tenets of the autoploid model.
Key words: autotetraploid, chromosomes, Machaeranthera pinnatifida, meiosis, polyploid, synaptonemal complex