Polar bears can breed with brown bears to produce fertile grizzly polar bear hybrids, rather than indicating that they have only recently diverged, the new evidence suggests more frequent mating has continued over a longer period of time, and thus the two bears remain genetically similar.
One of these subpopulations is in linkage equilibrium thus supporting the occurrence of frequent mating.
One of these subpopulations (X) shows linkage equilibrium and therefore evidence for frequent mating.
Frequent mating of females may therefore contribute to the maintenance of high sperm quality.
In one of these, parasite genotypic diversity could only be explained by the occurrence of frequent mating in T. congolense.
Frequent mating of females may therefore contribute to the maintenance of high sperm quality, as males remove sperm stored in the female before transferring their own sperm, although removal is not always complete.
To further test the level of linkage disequilibrium in the subpopulations, the index of association was measured (ISA; Table 2) and the largest subpopulation (X) shown to be in LE, thus indicating frequent mating within this population.
We see three ways to reconcile our observation of frequent mating between spores from different asci with the apparent rarity of outcrossing among the genome sequences of natural isolates.
T. brucei, along with Trypanosoma cruzi [9], are the only species of kinetoplastid parasite in which mating has been experimentally studied [5] and, after many years of controversy [10] [12], there is also strong evidence that some field populations of T. brucei undergo frequent mating, while others (the human infective subspecies) show evidence for asexual expansion of particular genotypes [13].
Our finding also has implications for the evolution of sexual recombination in eukaryotes; some African trypanosome species are evidently clonal and either never underwent, or have dispensed with, a sexual cycle (T. b. gambiense and T. vivax), whereas others clearly undergo frequent mating (T. congolense and T. b. brucei).
