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Twenty-one different caprine and 13 ovine MHC-DRB exon 2 sequences were determined including part of the adjacent introns containing simple repetitive (gt)n(ga)m elements. The positions for highly polymorphic DRB amino acids vary slightly among ungulates and other mammals. From man and mouse to ungulates the basic (gt)n(ga)m structure is fixed in evolution for 7 x 10(7) years whereas ample variations exist in the tandem (gt)n and (ga)m dinucleotides and especially their "degenerated" derivatives. Phylogenetic trees for the alpha-helices and beta-pleated sheets of the ungulate DRB sequences suggest different evolutionary histories. In hoofed animals as well as in humans DRB beta-sheet encoding sequences and adjacent intronic repeats can be assembled into virtually identical groups suggesting coevolution of noncoding as well as coding DNA. In contrast alpha-helices and C-terminal parts of the first DRB domain evolve distinctly. In the absence of a defined mechanism causing specific, site-directed mutations, double-recombination or gene-conversion-like events would readily explain this fact. The role of the intronic simple (gt)n(ga)m repeat is discussed with respect to these genetic exchange mechanisms during evolution.

The extent of parental care should usually increase with the benefits expected in tenns of reproductive success. In monogamous birds, parental care should therefore increase with brood size. Some recent studies failed to show such a relationship, and we wondered if this may be due to phenomena like extra-pair copulations and/or intraspecific brood parasitism, that could lead to nestlings which are unrelated to one or both putative parents. Thus, measuring the expected benefits by counting the nestlings may be misleading. In our study on parental care in the great tit (Parus major) we detennine parentage via multilocus DNA fingerprinting and show that parental care (measured as anti-predator nest defence) seems to be adjusted to the number of offspring fathered by the resident male rather than to the total number of nestlings.