il ' ii - Northern Research Station - USDA Forest Service

. RESULTS The heuristic search using equal weighting of the 300bp
data set returned 2 most parsimonious trees (369 steps;
Sequence• divergence based on pairwise comparisons of C.I. (excl. uninf.) = 0.461; fig. 1). All species of
the 19 Cynipini taxa (table 2) was less than 12 percent, _ Cynipini formed a monophyletic group; however, this
valueS typical of within-genus comparisons in other was usually not the case when multiple species from a
insects (e.g., Brown et al. 1994b). [Analysis of the 10 genus were present (e.g., with Callirhytis, Andricus and *
taxa for which 660bp was available was showed only a Neuroterus).
Slightly higher maximum divergence (table 2).] As
expected, highest divergence was found at 3rd positions DISCUSSION
in codons, where substitutions are often synonymous.
A+T base composition was 76 percent (table 3), a level The results of this preliminary analysis indicate that
typical for mitochondrial genes in insects (e.g., Liu and while the cytochrome oxidase I gene exhibits appropriate
Beckenbach 1992, Brown et al. 1994b, Spicer 1995); levels of variation for use in phylogenetic inference, two
base composition was particularly'A+T-biased at third aspects of sequence evolution in these genes, A+T bias
positions (95 percent; table 3), as has been found for and strong constraint at the amino acid level, raise
, _protein-coding mitochondrial genes in other Hy- concerns over inference methods. As 3rd codon position
menoptera [e.g., Apis mellifera (Crozier and Crozier substitutions are often synonymous, they are typically the
1993)]. This strong A+T bias has interesting effects on first to differentiate after lineages split. However, with
frequency of substitutions at these lower levels of effectively only.2 character states at 3rd positions (A and
divergence. We reconstructed the classes of substitutions T), instead of 4,reversals are likely with increasing
over one of the most parsimonious trees (see below), divergence time. Thus differences at 3rd positions are
excluding the relatively distant outgroups, using unlikely to be informative with highly divergent taxa.
MacClade 3.05 (Maddison and Maddison 1995). Con- By weighting 3rd codon positions changes less than
trary to the common discovery of a transition bias at low changes at other positions, one can favor conservative
levels of sequence divergence (e.g., Brown et al. 1994b, changes without discarding information from the less
• Liu and Beckenbach 1992), extreme A+T bias at third conservative 3rd positions. However, it is possible that
positions resulted in A_--_Ttransversions being equal in selection at the amino acid level may constrain evolution
number to all transitions combined (70 each), at 1st and 2nd positions such that differences accumulate
Table 2._Percent sequence divergence among 19 cynipini taxa. Values inparentheses are
. for 660 bp data set (with only 10 taxa).
• .
Sequence All 1st 2nd 3rd
divergence positions position position position
Minimum 1.9 (7.4) 0 (1.9) 0 (0.5) 5.9 (13.8)
Maximum 11.3 (13.1) 5.2 (8.3) 5.1 (7.1) 32.0 (31.2)
• Ip
Table 3._Percent base composition in the 300 bp region of COI in Cynipidae
" Base Codon position
All 1st 2nd 3rd
C 10.0 6.3 21.4 2.3
G 14.5 23.3 17.1 2.9
A 34.9 42.0 17.9 45.0
T 40.6 28.5 43.6 49.8
• A+T 75.5 70.5 61.5 94.8
I
243