TEMPO OF HYBRID INVIABILITY IN CENTRARCHID FISHES



(TELEOSTEI: CENTRARCHIDAE)




DANIEL I. BOLNICK1,2 AND THOMAS J. NEAR3,4




Asymmetries in F1 Hybrid Viability

Centrarchid hybrid viability differs between reciprocal

crosses of the same pair of species (F1 asymmetry). Of 18

species pairs for which reciprocal crosses have been done

(and viability is nonzero), 17 had significantly different viabilities

depending on which species was the female (or male)

parent. The relative strength of this asymmetry increased linearly

with time (Fig. 4), because the absolute difference in

viabilities was fairly constant and represented an increasing

proportion of the overall viability as the latter measure declined.


Asymmetrical F1 viabilities may also result from deleterious





interactions between the maternally provided oocyte



cytoplasm and the hybrid�s nuclear genes. Centrarchid hybrids



show aberrant timing of allozyme gene expression during



early development, even when the parental species have



identical onset of gene expression (Phillip et al. 1983). These



results suggest that centrarchid species have diverged in their



gene regulation mechanisms even while expression location



and timing remained similar. In many cases, hybrids expressed



maternal alleles at the normal time, but paternally



derived alleles were delayed, premature, or failed to be expressed



at all (Phillip et al. 1983). Less viable hybrids in a



reciprocal cross are generally the ones with greater paternal



allele misexpression. Whitt et al. (1977) suggested that the



greater effect on paternal alleles is evidence for cytoplasmicnuclear



interactions, hypothesizing that maternally encoded



regulatory signals are misinterpreted by the paternal allele.



If one species� gene expression is more sensitive to changes



in transcription factors, asymmetries will result.



One puzzling pattern



to emerge from our data lends some credence to a role for



cytonuclear interactions: using maximum body size as an



index (Page and Burr 1991), the larger species tends to be



the more successful maternal parent (Table 3). Of the 18



species pairs with reciprocal cross data and nonzero viability,



one pair had equal body size and nearly symmetrical crossing



success. Focusing on the remaining 17 species pairs (admittedly



not phylogenetically independent; Table 3), the larger



parent was more successful in 13 crosses and less successful




in four crosses (x12 5 4.765, P 5 0.029). We speculate that





there is greater disruption of paternal allele expression when



the paternal allele is from a smaller species, placed in an egg



with cytoplasmic factors encoded by a larger maternal species.



However, the cytoplasmic effect cannot be attributed to



differences in egg size, as egg size is not correlated with



body size (D. I. Bolnick, unpubl. data) and egg size differences



are not associated with inviability (Merriner 1971b).



We are working on expanding our dataset to include more



reciprocal crosses to test this pattern more rigorously


HAVING POSTED THIS ON CENTRAS NOTE THAT RES X BG SEEM TO BE THE EXCEPTION FOR SEVERAL REASONS INCLUDING NEAR SIMILAR SIZE OF THE TWO SPECIES AND DO CROSS WELL IN BOTH DIRECTIONS.



Last edited by ewest; 06/10/13 09:32 PM.